Usually a 120 gal tank is either 4 feet long or 6 feet long, depending upon the width of the tank. Assuming it's a 6 foot long tank, three MH bulbs should be fine.
The usual factors apply. You'll get more light out of a 250w bulb, but also more heat. In larger tanks it's also possible to use different lighting in different places. For example, you could use one 250w bulb and keep all the SPS and clams under that. It's more a balancing act rather than a hard fast rule.
If you have an unusual tank, something like 4 feet by 4 feet, then you might want four bulbs running over the tank, such as ricosuave suggests. Then again, you might want something different.
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