2nd axis is telling you if the bow is tipping left or right. Set it by holding a carpenter's level from limb pocket to limb pocket, get it plumb and check the sight's level. Adjust the 2nd axis untill it matches the carpenter's level.
3rd is telling you if the face of the sight is square to you. If you attached the carpenter's level to the bow, with tape, and you have 2nd axis set, draw and hold the bow plumb, now watch both levels and tip the bow up or down. Keeping the carpenters level plumb, your sight's level should also stay in the middle, if it's not, then likely 3rd axis needs ajusting. Your squaring the sight's face to you.