Here is what they say about the 3rd axis.

Another copy and paste.

3rd axis leveling
3rd axis leveling is the squaring of your level, so that the bow will be held in the same vertical plane for flat, uphill, or downhill shots. If your 3rd axis is not properly set, your level will lie to you, causing you to cant your bow when shooting up or downhill. Resulting in left and right arrows, no matter how good you executed your shot.
Leveling your 3rd axis must be done at FULL DRAW. Bow vises and bench jigs are not accurate because they don't take into account the torque in the bows riser when it is drawn. A sight that is squared at rest will not be square at full draw, when aimed up or downhill due to the torque in the riser. Both the Hogg-It and Hogg-It Hunter models have a 3rd axis adjustment that allow you to level up your sight without the bothersome task of trying to shim your bow mount.