In my personal opinion, the Arma 3 Preset is the best for gameplay as it's what I'm used to, coming from an Arma 2 background.
W - Move Forward
S - Move Backward
A - Strafe Left
D - Strafe Right
Q - Lean Left
E - Lean Right
Shift - Sprint
W+S - Toggle from jogging to walking, and vice versa
C - Change from rest to firing position whilst moving
X - Change stance from crouching to standing
Z - Change from whatever stance to prone
Left Control + W - Change stance (from current stance) upwards
Left Control + S - Change stance (from current stance) downwards
Left Control + A - Change stance (from current stance) leftwards
Left Control + D - Change stance (from current stance) rightwards
Left Control x 2 - Resting position/Gun on safety
Scroll Wheel - Weapon selection
I - Open inventory

This is mostly to be used when you're doing singleplayer scenarios, but it has its uses in multiplayer also.
` (Or whatever key is on the left of your 1 key) - Select all units in your squad. Upon selecting this, you have many options available. If you aim at an enemy, an engagement option will become available. If you aim at a position, you have the option to send your squad there, then select regroup for them to return to you. If you aim at your vehicle, you can tell your squad to embark it etc and so forth.
F1, F2, F3, F4, F5, F6, F7, F8, F9, F10, F11, F12 - Select individual unit in your squad
Middle Mouse/Enter/Space - Accept on scroll wheel or unit selection
1 - (When 1 or more of your units are selected) Move commands
2 - (When 1 or more of your units are selected) Targetting command. In multiplyer, it should create a red attack rectangle for your teammates to spot.
3 - (When 1 or more of your units are selected) Engagement commands. You can give the order for your units to fire at will or to hold fire until further notice. By activating this, your units will actively call out enemies they spot which you can see in the chat.
4 - (When 1 or more of your units are selected) Mounting/Embarking/Disembarking options. Tell your AI squad which unit to go on. Alternatively, you can also use `
5 - Status report. Upon using it, an AI command can send support your way in terms of re-armament, or medical support etc.
6 - (When 1 or more of your units are selected) Inventory & Actions. For example, you can command your unit to drop ammo, weapons and switch for alternatives if they run out. If they have mines, you can order them to place it at a certain location.
7 - (When 1 or more of your units are selected)Combat/Engagement status. Depending on which you use, your AI can act differently. For example, on stealth AI tends to prone and avoid detection and Combat will force your AI to be on alert and to use bounding cover.
8 - Formations. Pretty self explanatory.
9 - (When 1 or more of your units are selected)Team assignment. You can select teams to be in different colours.
0 - Response. I find it's pretty useless but nonetheless, it's to respond to your team leader for certain things.

Left Click - Fire
Right Click - Optics
Left Control + Right Click - Changing Optic mode. For example, if you have an ACOG, you can switch from the 4x magnification to the RMR ontop of your weapon for close range combat.
R - Reload
F - Firing mode toggle, for example if you have an underbarrel GL attachment. Switch from Full auto to Semi auto. For Guided Launchers like the MRPL, it switches from DIrect firing mode to Top-Down, increasing the chances of hitting your target.
Page Up - Increase Zeroing (High Magnification Optics only)
Page Down - Decrease Zeroing (High Magnification Optics only)
T - Target Lock-on/Automatic Zeroing. This only applies to missile launchers.
N - (Magnified) Toggle Night Vision/Thermal optics. This only applies to specialist missile launchers and specific sights, like the Nightstalker.
Control + G - Changes throwable equipment.
G - Throws throwable equipment, e.g. Grenades, Smokes, Flares.

Sub-Machine Guns - Guns that have a barrel less than 10", not offer much in firepower but makes up in weight and the amount of munitions you can carry as they use low damage calibres such as 9mm, 45ACP, 9x19mm


Assault Rifles - Guns that uses a 5.56, 6.5, or 7.62 calibre with a 30 round magazine and a barrel length of no more than 10". Carried by nearly all soldiers. Examples include the MXM, the Katiba, the MK20 or the AK12 (Apex DLC). Travel distance for these rounds range from 200-500m, perfect for the everyday shooter.

Carbines - Basically a shorter assault rifle. More suited to Close Quarters Combat.

Marksman Rifles - Guns that inherently use 7.62 calibre or more with 20 round magazine and a barrel length more than 10". Marksman specialises in engaging distance at range where the conventional soldier can't utilising a medium or high optic scope.

Support Weapons - Guns that can use any calibre and have a magazine size of 100 rounds or more. Like the title, their role is to support by providing and making a tactical advantage for example, concentrated suppressing fire. Examples include the MXM LMG edition, Navid, SPMG.

Sniper Rifles - Guns that use specialised high calibre that can engage enemies up to 1000m away. Examples include the GMG Lynx or M320.

Launchers - A heavy piece of weaponary usually used to take out vehicles. Selection of armament includes High Explosives or Anti-Tanks, HE obviously used for infantry whilst AT is for vehicles.

Handguns - A backup weapon to a traditional rifle for use in CQB. Ranges only up to 50m-100m. Though the calibre is not particular damaging at distances, it is however lethal close-range to body hits. Kevlar easily stops the rounds.

M - Opens/Closes map

• Double click - Creates marker
  
• Left Control + Left Click + Drag - Draws on map
  
• Shift + Left Click - Sets a personal marker for you where you can view once out of the map

[ ] - Opens your left or right panels.
Options:

• GPS - Shows your location
  
• Mine Detector - Detects nearby mines & explosives, will start beeping when you're in the precense of one. To spot them for your other teammates (and yourself), press T.
  
• UAV - If you have a UAV terminal and an UAV connected, you can switch the panels to the live feed from your UAV targetting camera or pilot camera.
  
• Seats available (in vehicle) - Will show you how many seats are available in a vehicle.

Left Control + G - Switches grenade type

First Aid Kit - Heals teammates, is perishable and will be deleted upon being used
Medkit - Heals teammates, has infinite uses and can heal people faster if you have the medic perk.
Toolkit - Repairs Vehicles, disarms mines.
Smokes - Coloured smokes can be used as a tool for signalling, alternatively use white smoke to conceal your movements from enemy.
B - Switches to your binocular, or equivalent like rangefinder or laser designator
Grenades - Offensive or defensive grenades that are used to clear clusters of enemies.

Thanks for 500 views, this was my first attempt at going about creating a guide for a game.
If there's any queries or questions, or suggestions for new guides for me to make (Trust me, these 4000 hours aren't for show) please do leave a comment.