In Mission 12: Old Comrades, grab the different weapons dropped by enemies and get a kill with each. There are many more than three weapons in this mission. From the first weapons box, you can choose from three weapons — grab the M19 (it is the rarest drop by enemies in this mission).
At the end of the mission, you will enter a van where you find the M91 Machine Gun and RPG-7 Rocket Launcher. That is three weapons automatically given to you that you do not have to worry about — so you really just need to find five from enemies, which should be quite easy.