How this rotation runs
This is a push rotation. Every 20 minutes the guard on break walks to the deep end and takes over. The deep-end guard moves to the shallow end, and the shallow-end guard goes on break. Each guard watches for 40 minutes (deep, then shallow) and then gets 20 minutes off.
Put the stand that needs the freshest eyes first, since that is where the guard coming back from break goes. For most pools that is the deep end or the diving well. If your 3 guards cover only one stand, each guard is up 20 minutes and down 40.
Variations
Other setups
Rotation limits from the CDC Model Aquatic Health Code (6.3.3.1.2). Your facility's rules and certifying agency come first.