Oryginal player receives 36 minutes
Backup player receives 10 minutes
Reserve player receives 2 minutes
Bench player receives 0 minutes
If player is set as original + backup on the same position, then minutes for backup player are cut to 50%, so he gains 36 + 10/2 = 42 minutes.
If player is set as original + backup + reserve on the same position then he receives 43 minutes.
If player is set as original + reserve on the same position then he receives 38 minutes.
If player is set as backup and reserve on few positions than minutes are cumulative without cut.
I'm not sure what happens if player is set as original on one position and backup or reserve on other positions.
I tried that the last time I had a walkover.
Player A: Starting PG / Backup SG / Reserve SF
Player B: Starting SG / Backup SF/ Reserve PG
Player C: Starting SF / Backup PG / Reserve SG
And they all got 43 minutes. No way that I know of to game the system to get 48 (though I didn't experiment with setting one player as Starter at one position and Backup at two or more others).