Electric peeps - ballast question

[Myka]

I think I know the answer, but I want to confirm.

I have a couple of Workhorse 5 ballasts in a single box. They are 128 watts "maximum power output" and are described to operate 1-3 39W T5 bulbs each.

Does that mean if I run:

1-39W bulb it will be over-driven at 128 watts?
2-39W bulbs will be over-driven at 64 watts each?
3-39W bulbs will be operated at 42 watts each?

Or is it not that simple? I imagine the ballasts will not pull exactly 128 watts...?