To compare all three - stand in one spot.

Sight on an object fairly close, then farther away, then farther still. Not the readings each time for each compass. Note any differences.

Now do what unimogbert said to do. If you have a protractor you will know the bearing. Again, use all three compasses. With luck, one's right.

My dad had a compass course in our backyard. After many tries the best I could do was +/- 2 meters over about a half mile. This was using bearing and pacecount only. He had it down to 2 feet at one point.