Who do you think would win? Or rather, who would you want to win? If these 3 empires went to war I think the Nerubians would have the advantage of having a massive underground network. The other two have little holes, but nothing like the Nerubian empire. The advantage of the Qiraji and Mantids would probably be having a lot more flying units, but again, the Nerubians can simply burrow and bottleneck the incoming forces. Nerubians might have less numbers (no source to confirm, just a hunch) but they seem pretty well-rounded in magic, so that could help for taking out swarms and getting past thick chitin.