Checking the bubble should become part of the shot routine, it can show problems even when you have perfectly level and comfortable footing, hills or anything not level near or behind the target can play tricks with your brain and make you cant the bow. If you get it into the shot routine to check/adjust to the bubble your life becomes better.

There are some easy, though not perfect ways, to check the 3rd axis. I've used a doorway to do it. Rest the bow against the doorway (first make sure the doorway is plumb!). Make sure the sight bubble is level (2nd axis I believe). Now assuming your doorway/wall will allow this, rotate the bow keeping it up against whatever you're using, if the bow stays square to the floor, the bubble should stay centered.

Not all sights have a third axis adjustment, but they are nice. Even if you can't do 3rd axis you may find you need to adjust it to be on on the second axis!