Changed a broken screen and touch area doesnt work proper

A 3g screen will not plug into a 3gs mother board and visa versa. The connections are different, 3gs screen connections has two rows of prons/teeth with a black space between, 3g has 3 rows of prons/teeth and no space. Most likely the connections popped loose when lowering the screen back down to the phone, double check that the connections are flat and secure. Sometimes you'll get defective screens, make sure you buy original oem pieces and not generic "clones" although they work fine. Two types of clones are called High copy A and High copy AA. Single A is best "clone" quality. AA is good too, but not so much. Original "Just like the one you bought from store" pieces would be adveritised. Sellers say their parts are oem but are most times clones. Make sure the connections are on and you talk to sellers before purchasing.