Ok how about a quick math lesson. When building a T copter should the motors shaft be
Equal length from one another.
Say the front two are 9" apart should the rear motor be 9" from the left and right motors
Or a piece of string should work. Tie a loop in one end, and slip it over the prop shaft of one motors. Stretch this out to the prop shaft of another motor, and make a sharpie mark. Then swing it around to the third motor, and check against the sharpie mark. You can repeat by moving the loop to the other motor's prop shafts.
If the measurements are all the same, you should be good.
I had this very same question a few months back when I was building my first T-copter. In my search, I stumbled across this RCGroups thread T-style tricopter design rules here and in that thread, someone created this wonderful T-Copter Dimensions Calculator that works really well for figuring out all of the dimensions and where the CG needs to be.