The 3n1 (or 4n1) control unit on the copter has 2 pot adjustments. One for gain and one for proportional. The Proportional pot is the adjustment for the motors. Center your trims. With the heli battery unplugged, turn the pot adjustment clockwise. Use tiny turns. Turn pot so it barely moves. Plug battery in and test copter. Keep adjusting until either the nose doesn't drift or until a clockwise adjustment goes right, then counter-clockwise goes left. The trim will work better after that. If you download the manual, it covers that specific problem.
http://www.e-fliterc.com/ProdInfo/Fi...CX2_Manual.pdf