Is there much of a reason to use T3 when bulking? Probably not in most cases. The whole idea of bulking is to consume more energy so you may grow, T3 thrives off of burning energy, it directly feeds off of ATP. In the past I ran low doses of T3 while bulking and while it didn't kill me or anything it didn't seem to do a lot for me either.
If you're bulking and getting too fat you simply need to eat less...this shouldn't be that hard to figure out. If you're worried this is going to hamper your gains don't, get that thought out of your mind because the excess food wasn't helping you if the excess calories were being stored as fat...I'd put every dime I have on most guys eating way more than they actually need and thinking they're actually doing themselves some good.
T3 making you flat...if there is not enough energy to support the T3 you are taking then it will take what it needs from the muscle tissue and you will look a little flatter...this will not happen overnight. However, I really don't think most understand what flat truly is, so many get so used to being swollen or bloated they assume this is full and when it goes away they're flat...this is wrong.