What size schedule 40 conduit should I use to run three circuits through? (two 20amp circuits and one 15amp circuit.) The distance is 75'. About 60' is to be a surface run and the remainder will underground. I just don't want to have run three separate conduits. Thank you

3/4 inch would work fine.

I'm curious about what you are trying to do and why? I want to understand the installation.

1) When you say "Surface" exactly what do you mean?

2) Have you considered using a down stream panelboard rather than running six conductors and 3 grounds inside conduit. That way, you could tap off the main panelboard and run one set of conductors to the downstream panelboard.