Questions on the line of scrimmage and lining up

[devilsadvocate]

1. Is the offensive line required to be balanced?
2. When are you considered to be lined up on the line of scrimmage?
3. Three running backs line up (set) behind the quarterback. All at once they move to a trips formation then get set for one second. Illegal motion or legal movement? If this is illegal, how could the running backs move to that formation legally (and try to catch the defense off guard)?

Thanks.