I'm playing my first game now on normal difficulty. Like you, I have 3 bases and 2 research labs. I think I'm in the mid point of the game. Cruisers are appearing and I have my first marauder flying around. I try to keep my research balanced between offensive and defensive technologies. And I try to keep several research projects going at once. You do know that each additional lab person added to a research project is less effective than the previous one, so the spreading them around to multiple projects is more efficient overall.

It could be your problem is with the ground combat strategy. I learned a lot from watching youtube videos people have made. I've never played tactical simulations before so this was very helpful to me. Also, I found my combat losses dropped substantially after getting jackal armor for everyone.

My basic strategy for ground combat is to have a diverse set of soldiers; 1 machine gunner, 1 rocket launcher, 1 sniper, one scouting guy with a shield, and the rest riflemen. Keep them close together, keep a soldier watching the rear and any unexplored areas, advance slowly leaving TUs available for reaction fire, and send a high TU scout out in front to explore but bringing him back to the main group before the turn is over. Then advance the whole group and repeat. I found shooting the aliens at range was better than getting close to them. Remember to make use of grenades and smoke grenades. And you can use your rocket launcher to take out cover that is protecting an alien. Just watch how people do it on the you tube videos; they aren't all great at combat but you still learn things from them.