Dispersed, asymmetrical bases on each country.
Place a few bases in the middle of of each country's side of the map and make them uncapturable. With the idea that any country can get to a fight against any other country at any time.
So maybe have 1 or two bases each country scattered amongst each of the other two countries regions on the map. Uncapturable.
Not sure how game logistics work - just throwing out ideas that might make it easier for players from all counties to be able to engage each other. A lot of times the main battles are between two sides and the other side can't really get there easily.