assuming you're playing a 2-1-2
there are a few ways you can play it.
you play 1 towards window or doors and 1 towards plat
you play 1 towards doors and 1 towards window
you play 1 towards doors and 1 in the back of site
there are other ways to play it, but the main point here is that you want to obviously be playing long range angles, which means not playing car. on top of that, you need to make sure you don't get caught out in the open trying to throw a nade.
you really need to have a good bit of teamwork here to hold very well, and it's a lot about landing your first shot, however you should consider what your goal is as a B player
delay for rotates
not die (obviously delays for rotates)
get kills
now with setup 1, you're committing 1 player to die, hoping that you can land your shots and you don't get unlucky and instantly die. you're allowing 1 player to survive by playing window, getting 1-2 and then falling off and hitting in later with the team. this means you're going to have less kill power, but more retake power
with setup 2, you're playing full on retake. you try to get some damage and get some kills, but primarily you're allowing for a good retake after you get 1-2 kills hopefully.
with setup 3, you're mostly comitting the same thing as in setup 1, however you have a bit of a crossfire as they run back into site.
I'm rambling a little bit here, but basically just decide on one of the 3 setups, then decide on your nades. personally, I would suggest you either run nade/flash/smoke and armor, or nade/flash/flash and molotov. obviously by using a molotov you're very easily going to be able to stop that B rush, but you're going to have a harder time on retakes at a, and you're going to have a pretty tough time holding late takes because molotoving B after you see them hitting in can be pretty hard.
so basically if you're super worried about the B rush, run setup 1 with the guy on plat having a molotov, and the guy in window having armor, or the guy in door having 2 flashes and a nade. by doing that, you have the guy on plat molotov off the tunnels and the guy at doors can spot in.
otherwise, you can run setup 1 with the guy on plat having armor. he basically just holds plat trying to frag out, and the guy in doors is going to spot them out as they run in, then double flash in front of box so that all of the Ts are blind and won't be able to fire back. then your mid guy rotates into window and you have 3 guys holding towards B together
anyways, sorry but basically I coulda made this way shorter, but im not going back and deleting it because fuck that.