Noob question: worth the half point?

[VarlosZ]

This week, I can get Giants +3.5 -118, or +3 +101. I understand that that's a very important half point, but it seems like you're giving up an awful lot to get it.

How often would the Cowboys have to win by exactly three for the half point to be worth it? And about how often will they win by three? Thanks.