Metal gear solid 4 act 3?

May be...

i'm at the first mission on act 3 i follow the resistance members then i see the pmc troop on their way so i try another one and that one get arrested bye a pmc troop

Just stay behind the res guy in full camo. have a silenced sniper and snipe the pmc before they see you. as long as you stay close to the res guy youll be good. and if he gets scared off, it's easy to find him just look on your map.

Kill the guards with a silenced gun or tranquilezer but don't let the resistance guy see you or he will shoot you and run away.

use your tranquiliser if the resistance guy gets arrested, also dont let the resistance dude see you doing it else hel shoot you

Kill the guys who arrest him with a sniper or something, don't let anyone see you kill them tho, especially the res. member.