If you’re even thinking about it now, that usually tells you something.
I didn’t add a third during my build because I thought two would be fine for a “simple rectangle.” What I didn’t fully account for was how benches and step areas mess with flow. Once everything was up and running, I had one stubborn corner where fine debris would just hang out. Not dramatic, just annoying enough that I noticed it every week.
I managed it by tweaking eyeballs and slightly increasing run time, but it also meant after heavier bather load days that area clouded up first and my filter pressure would climb a few psi faster than expected. Nothing catastrophic, just not as balanced as it could’ve been.
If plumbing is still open, adding a third and valving it so you can control or even cap it is cheap flexibility. You don’t have to blast it full time. Sometimes just having the option to redirect flow seasonally makes a difference, especially if wind consistently pushes surface debris one direction.
Once concrete is poured, you’re basically married to the layout. Extra return now is a small line item. Retrofitting later is a jackhammer. If your pool has any ledges, benches, or odd angles, I’d personally overbuild circulation and dial it back rather than wish you had more movement later.
