Probability question concerning odd makers and spread

[iceblits]

Homework Statement

A bookie will pay you 6$ for every 1$ bet if you correctly select the winners (adjusted by a spread) in three games. What is the bookies expected earnings? (spread is how much the team wins by, and, when added to the underdog's score, should result in a tie)

Homework Equations

E[x]=sum(x*f(x))

The Attempt at a Solution

I'm having trouble getting started and making a probability density function for this because I don't really know how to deal with "the spread". If there was no spread would the pdf look like this?:
X=number of winners selected...money earned by bookie...probability
0.......... $3........ 1/8
1.......... $-3......3/8
2.......... $-9......3/8
3...........$-15......1/8

and then his expected earnings for this would be 3*(18)+(-3)*(3/8)+(-9)*(3/8)+(-15)*(1/8)

 

[Adrmay]

=-3.375But I'm not sure how to account for the spread in this. Any help would be greatly appreciated!