From Another Forum (NCAA Question)

[MNBlue]

This isn't standard base mechanics for NCAA - at least one person starts on the line every pitch. With runners on 1B and 3B, U3 should be on the line, squared to the plate, 6-10" from 3B. It is permissible to start a step or two off the line in order to be able to see the plate after the pitch.

For checked swings this is the order of who should be gone to for help:
Both umpires on the line, go to the open side: U1 for a RH bater and U3 for a LH batter
One umpire inside the diamond, go to the umpire on the line.