First, let me say that I love the idea of a 3 action system. I think, if done well, it will influence both the tactical game play and the class design far into the future. I do think that it would be better if characters got more actions with higher levels, say 4 actions at 10th, 5 at 20th. This would effectively describe how higher level characters can do more things in combat. Spells like haste or action surge type class features surely offer additional actions, if temporarily.
I think really big spells or martial maneuvers need to take 3 actions. Those 3 action options open up a lot of possibilities. I'm assuming movement is restricted to 2 move actions per round to stay in line with the double move of pretty much all d20 systems.
I say skip the stacking attack penalty. It's not needed. If the maneuvers are better then making individual attacks then it would work. For example, making a single attack just deals weapon damage. Using a 2 action maneuver to attack adds Str bonus damage or allows you to gain some AC. Just don't go 4e on us. That was too much.