It goes like this:
Round 1: Pick Attack, M+A or Support. Let's say Attack.
Round 2: Pick M+A or Support. No Attack available. Let's say M+A this round.
Round 3: Pick Attack or Support (M+A unavailable as it was used last round).
And so on. Pretty much exactly what you thought was going on (after round 1 you pick one of two Actions). Of course some Events can prevent you from picking one of the remaining options (usually Attack is the one that becomes unavailable) so there might be times when you have no choice, you have two Actions in hand, but Event prevents you from choosing one of them, have to use the only one available.