From what I've experienced with mine and from what I've read in the forums, the most likely causes of poor yaw control would be a bad tail motor or the proportional and gain adjustments on the 3-in-1. I don't think a different transmitter will help much with that, unless some expo on the rudder helps. There could also be a problem with the 3-in-1 itself I suppose.

A different transmitter can help with things like allowing you to use expo to reduce the senstivity of the controls and adjusting throttle and pitch curves.

I can tell you that even a fairly new tail motor can be bad. A hard tail rotor strike killed a new tail motor on mine and made the tail very hard to control. The motor still ran but just wasn't strong enough to control the tail well anymore.

I never had to adjust the proportional and gain on mine when it was new. It worked fine right out of the box, but if I were having trouble with tail control I'd try that.

You should have pretty stable yaw control with the stock setup. Mine had some drift when climbing or descending rapidly, but was easy to compensate for. In a hover it was quite stable but would require small trim adjustments from time to time during a flight. I would make sure everything is ok with the stock setup before doing any upgrades.

I put a G90 on mine and felt that it didn't make a whole lot of difference in tail stability, but then mine was fairly stable to begin with. Then I put a direct drive GWS motor setup on it and that made a big difference. The tail stays straight on fast climbs or descents now. The DD setup and the G90 together are working well for me now. I like being able to adjust the gain right from the transmitter.

Hope this helps.