You should replace the basic fins with canards to make up for no engine gimbal. If you still have control issues on the other 3 stage variant, you should instead have a swivel engine on the middle stage. Of course you could always add a rediculous amount of reaction wheels, but that would be impractical since the craft has only 1.25 TWR. If you are short on money you could just do 'more boosters' and use a swivel for main engine with four canards. Believe me I used to have this problem all the time when I first started: once the rocket starts to tilt and can't recover it's all over. I hope that this helps with your issue!