This is easy in principle. What you need is a cascade of radio triggers.
For clarity, I'll assume that you are familiar with radio triggers. If you're not, no problem - just let us know and we'll start from the beginning.
So to start you need to set up 3 radio channels
"1" setRadioMsg "Give orders to 1 Platoon"
etc
Now 3 triggers
Activation: Radio Alpha
On Activation: [] exec "alpha.sqs"
etc
(If you're clever with scripts you could probably use just one script and pass alpha, beta, charlie as variables, but that's another story)
alpha.sqs
; this is a script to allow the player to send commands to 1 platoon
"1" setRadioMsg "Attack!"
"2" setRadioMsg "Flank left"
"1" setRadioMsg "Flank right"
(etc.)
;this variable is required to control the various radio alpha triggers that are in the game. Use other variables to turn triggers on an off, so that at any one moment only one radio alpha trigger can be activated.
1command=true
@1command=false
"1" setRadioMsg "Give orders to 1 Platoon"
etc
"4" setRadioMsg "Null"
etc
exit
Now you need a bunch of triggers
Activation box: Radio Alpha
Activation: this and 1command
On Activation: player sidechat "1 Platoon attack!"; 1command=false
This is only a sketch of what you need to do, syntax and details may need some tweaking. Anyway, I hope it makes sense. Ask if not.
edit: I don't know much about MP but I do know that it's not script-friendly. You'll probably need to pass variables between the clients and the server. It might be easier to use gamelogics and looping waypoints rather than a script.