Well, if you just want an average dividend, then you simply add them together and divide by 3. But I assume you actually want to work out the average return per $1 invested (just as if you had been backing just one horse)

Do you bet the same amount on each horse? If you do, then you can simply do it like this:

Example:
1. $8.90
2. $6.10
3. $10.20

Total is $25.20 divided by 3 gives an average dividend of $8.40. But to work out the average return you'll get for your dollar, you have to divide by 3 again. In this case it's $2.80.