The key aspect for Stage 3 is to expand your empire via military force
Conquest
Stealing other empire's planets or enslaving them is a way of conquest.
Things you should know:-
1.
stands for DP (short for damage protection). If you are under DP, no one can attack you. 2. You get DP if you lost 50% of your ships or a large amount of your planets
Advise ?
1. Enslaving is a good way of expanding.
( All slave empires pay you tribute, the more slave empire you, the more credits you have )
2. Read up on how battles work ( Manual > Useful Guide > How battles work )
3. Always have at least 3 stacks (different types of ships)