The only airplanes that I have seen the took any up thrust were bipes and then only around 1/2 degree. 3 degrees is extreme. Tail heavy AND up trim do not make sense at all. I would suggest going back to square one. Level out the fuselage using the canopy frame as a reference. Set the stab to zero, engine to zero but with 3 degrees right thrust and wings at .5 degree positive. Set CG between 28% and 30% MAC. The Yak is a bit short coupled so keep elevator throw to around 15-20 degrees for your first flights.