hmmm, something like this could work:
make a trigger, and put this in its condition:
!player hasWeapon "M16"
and then put in the activation of what U want it to do
then, make another 2 triggers which each have 1 of the following in em:
1 with
!player hasWeapon "M60"
and 1 with
!player hasWeapon "M21"