Gyros are needed, you would need at least four (one for each motor, one for yaw)
Three motors is actually not that hard. The yaw servo tilts one motor to control the yaw, which is pretty easy. The ros should take care of the stabilization.
Basically, with a computer radio ( dx6i or dx7), four gyros, and a lot of servo v-tail mixers, you can make a stabilized tricopter without any custom electronics.
Also, brushless motors will be much better and cheaper.