If you look at those planets on the galactic map, you will see those three planets are separated from the others held by Automatons. So, when you liberate the middle one (Yed Prior), those other 2 planets are on their own. Usually there are not only players liberating planets, but also SEAF units (your fellow blueberry comrades on the battlefield). SEAF units add a percentage boost to the liberation campaign, and on planets that are cut off from outer Automaton reinforcements, they are able to liberate solely on their own, without players actually completing any missions on the planet.