How many volts should be across each line on a 3 phase motor?

Three phase means just that- three phases in the windings of the motor. Therefore three leads (outputs) are used to supply power. A lot depends on the number of wires coming out of the motor. In large industrial motors, you can have 9 leads. The wiring of the motor depends on the application- the motor will either be wired in series (leads 1-4-7, 2-5-8 & 3-6-9 combination) or parallel (4-7, 5-8 & 6-9 with the first three leads connected separate).
To change the direction of the output shaft, simply reverse any two of the three leads.