Do I need third axis if I only shoot indoors or flat outdoor target?

No. It only affects inclined shots. Set the second axis and move on; third axis becomes worth the effort the first time you shoot a hillside course or hunt from a stand.

Which way do arrows go if third axis is wrong?

Left or right on uphill and downhill shots only, growing with the incline and the distance. Flat shots stay true, which is the giveaway that it's the third axis and not windage.

Can I calibrate it without a jig?

Yes, with a plumb line and patience: hold the bow tilted steeply up and down while keeping the string parallel to the line, and adjust until the bubble stays centred. A jig just makes it faster and repeatable.

Third Axis

The third axis is the sight housing's rotation about a vertical line — its tilt toward or away from the archer. When it is set correctly the bubble level stays truthful on steep uphill and downhill shots; when it isn't, the bubble reads level while the bow is actually canted, and arrows go left or right on inclined shots only.

Details

On level ground a mis-set third axis is invisible: the bubble is checked with the bow vertical and the sight pointing straight ahead, so the housing's fore-aft tilt has no effect. Tilt the bow up or down a steep hill and the geometry changes — the bubble now measures a slightly different plane than the riser, and the archer levels a bubble that is lying. The error is proportional to the incline, which is why third axis is a 3D, field and mountain-hunting concern rather than a target-range one.

Calibration needs a true vertical reference and an inclined view. The common method: set the second axis first, then mount the bow in a levelling jig or hold it against a plumb line, tilt it 45° up and down as if shooting a steep shot, and adjust the third-axis screws until the bubble stays centred through the tilt. Sight manufacturers each have a slightly different adjustment mechanism, but the principle is the same — the housing's fore-aft tilt is being matched to the riser's.

Field checks are useful. On a steep shot with a known-good sight, aim at a plumb line hung down a hillside: if the bubble is centred and the pin tracks the line, the axis is right. If the pin drifts off the line as you tilt, it isn't.

Third axis drifts less than second axis, but it can move when a sight is knocked or re-mounted, and some sights let it creep as screws loosen. Re-verify before any event with steep shots, and log the setting date with the rest of the sight setup.

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Common questions

Do I need third axis if I only shoot indoors or flat outdoor target?: No. It only affects inclined shots. Set the second axis and move on; third axis becomes worth the effort the first time you shoot a hillside course or hunt from a stand.
Which way do arrows go if third axis is wrong?: Left or right on uphill and downhill shots only, growing with the incline and the distance. Flat shots stay true, which is the giveaway that it's the third axis and not windage.
Can I calibrate it without a jig?: Yes, with a plumb line and patience: hold the bow tilted steeply up and down while keeping the string parallel to the line, and adjust until the bubble stays centred. A jig just makes it faster and repeatable.