So 'you may destory foundations in addition to discarding momentum to make multiple copies' means what exactly?

I can destroy three of my foundations to get those copies or I do both; discard the momentum and destroy three of my foundation?

It means you can destroy foundations as well as discard momentum, and any card that goes to your discard pile as part of this cost becomes a multiple. So you can choose to discard 3 momentum and destroy no foundations, destroy 3 foundations discarding 0 momentum, or anywhere in between, so 1 momentum 2 foundations etc.