Best use of system

If you have coverage in all the areas that need it, and a good backhaul connection, then you don't really need to change anything. As for the satellite you haven't deployed, maybe try placing it in the same room as the router, push the sync buttons on both the router and that satellite, and see if they sync up properly or not. If they do, then you can deploy the satellite wherever you have the weakest coverage.

If not, maybe try a factory reset on the satellite, and retry.

If that still doesn't work, you can go into the Orbi router settings under advanced > advanced setup > wireless settings, and click the generate button under backhaul password, and then repeat syncing again. If you do this, you will need to resync all of your satellites, as they will no longer have the correct password for the backhaul connection. Some people have had luck just by syncing the problem satellite first, and then the satellites that were already working.

Also, I'd make sure you do NOT have any wired devices connected to the satellite while you attempt to sync it. This goes for both the problem satellite, and any existing ones that currently work if you generate a new backhaul password and need to resync them all. Once a satellite is synced and working correctly, then you can plug in any wired devices, and move on to the next satellite. Sync them one at a time, and once they're stable, then repeat for the second and third satellites.

If all else fails, you can factory reset the router and all satellites, and then sync them one at a time.