Given your statements:
1) check the trim on the xmitr (bottom, horizontal, trim on the left joystick).
2) Look over the 3-in-1 board for obvious physical damage (knocked a trim pot (three on the front of the board) loose, or burned components).
3) try adjustment of the gyro gain.

You didn't say if you had had any hard touchdowns or crashes.
You said the tail motor was O.K., but you did not say how you know.
Unplug the tail motor from the 3-in-1 board, apply power from two AA batteries in series (polarity is not important) to the tail motor. You should see the tail rotor jerk or spin. if so, tail motor is most likely O.K.
Plug tail motor back into 3-in-1. Unplug one of the Main motor leads. Turn the xmitr on. Plug in the flight battery. Pick up the heli by the landing struts. Push the throttle/yaw control (left stick) to full throttle and then full Right yaw. If the tail motor does not twitch or turn, wag the tail counter-clockwise fast, bring the tail slowly back to starting point, and wag rapidly counter-clockwise again. The tail rotor should start turning during the wag. If not, suspect the Gyro gain/sensitivity and/or the 3-in-1 board.

That's the best that I have. There are a lot of guys out there with more expertise than me. Maybe they can help more.