Measuring multiple revolutions reduces the inaccuracy from getting the start and end points on the wheel wrong in measuring the rollout.

Let's say you've got a wheel that's exactly 2200 mm in circumference. You measure, and you get the start and end points off by 1% each, in opposite directions, so you measure it as 2244 (or 2156). Your computer is going to be off by about two percent. If you measure three revolutions, and you're off by 1% of a wheel circumference, you'll measure 6644 mm, and your computer is going to be off by about 0.6%.

This is a pretty standard error reduction technique, used when the human error is roughly constant, and measuring a big quantity (or time) causes it to be a smaller fraction of the total.