Partly because one member doesnt do so good, but most of the time the 2 other members can carry the last player. Thing is when we go against a team full of tanks/bruisers (ex mumu, leesin, malph) we get destroyed.

Question is, how exactly can you counter tank/bruiser teams when it's hard to focus down the weakest when they're all tanks?

Win lane, gank often, control your jungle. Teamfights are won before they start.

We've tried that, against uncoordinated triple tanks it worked fine, but when all three tanks move together anywhere outside of the laning phase... cant even fight em

So, you have a rather bad player, and the other team is coordinating better. You may need to replace him and y'all need to be talking about everything over skype or vent.

Also, why are you so hung up about the enemy playing bruisers and tanks? That's the meta in 3's, lots of bruisers.

bench your worst player, find another
if he is your friend and you don't want to bench him, create another team without him