[kyro]

I did these calculations solely to make sure I know what I'm talking about. If I am way off here, please someone let me know where I went wrong. Thanks!

Bet $100.

At 3.5, you're betting $100 to win $84.75.
At 3, you're betting $100 to win $101.

If you bet the -118 line, your EV where x = prob. cowboys win by exactly 3, y = prob. cowboys win by more than 3 and z = prob. cowboys win by less than 3 (or lose outright) is

(y)(-100) + (x+z)(84.75) = 84.75x - 100y + 84.75z

With the +101 line, it is:

(x)(0) + (y)(-100) + (z)(101) = -100y + 101z

Set the lines equal to each other to see the break-even point and the answer is x = .192z

So the Cowboys would have to win by exactly 3 around 1/5 as often as they win by less than 3 (once again, or lose outright).