you can make a level 3 cluster, but yes it will be hard, and there is no way you will be able to explore for anything past your 50-60th planet. you will have to fight for those last 100planets, or clusters. and is it very possible to make a C3, look at the most planets in the ranking page! and you can actually explore to 16 colonies, and if you attack someone when you have 14 colonies you can win 3 more colonies, putting you at 17 total colonies. but with 15 colony limit, it is still very possible to make a C3, in fact terrans have the highest max colonies in the game, being 15