i have a 3 tank setup, left and right inboard tanks are setup via taps, on and off only for left, and the right side, is switchable between right inboard and right outboard, ( or reserve as i call it), and off. the engine return feeds the right inboard main tank. when i fly with 3 full tanks, i take off on right main only (with the engine return otherwise it will overflow) , and fly for about an hour, this drains about 18 ltrs from it, then ill turn that off,
Then i run on the right outboard reserve tank until its dry, as in, when the light comes on in the header. (the reserve tank has no level gauge) the inboard right tank is almost full again as well.
i have noticed that when i open the right main tank, after the light in the header tank goes on, it takes a while to go out, sometimes quite a while, but thats usually fixed when i open the left tank as well, and fuel can then crossfeed between left and right inboard tanks to even out their levels. if i get a bit worried, or annoyed about the header tank light still being on sometime after the tank change, i will fly unbalanced for a min or 2, this fixes it pretty much every time.