The three in question (First Light, Bastion, & Skyshock) are not available through classic 3 v 3 (skirmish & salvage) nor classic ffa (rumble). They only appear randomly through classic 6 v 6 (control, clash, & combined arms). To my knowledge, these are combined arms maps, so you have to play classic 6 v 6 for enough matches that it gives you a combined arms match. The upshot is that the only combined arms maps were/are the three in question, so you should get at least one of the three during your first combined arms match in classic 6 v 6.