Preventing multiple promotions from driving a line's price negative

 I have a simple requirement.  We need the ability to ensure that if an order line item has multiple promotions applicable, then the end-result should not drive the selling price negative. Here is an example:
I have the following three promotions defined:

Promotion 1:  50% Off
item: C452W
Discount of 50%

Promotion 2:  Free For 3 Months
item: C452W
Discount of 100%

Promotion 3: 10% Off for 3 Months
item: C452W
Discount: 10%

Assume that all three promotions are automatic, and promotions 1 and 2 qualify to apply on an order line. I would want promotion 2 to apply. But how do I prevent the selling price from going negative, since promotion 1 also applies? I dont care either if promotion 2 either gets applied at 0%, or not get applied at all - either options are ok with me.