Well I'd keep doing what your doing for the moment but be very wary of your real odds. You should make sure your going to get at least evens on your shortest priced runner. A 3 horse bet would mean all of them are paying at least $6 min. If it's less, you are accepting odds-on. Not good in my book, but others are OK with it.

Swings and slides. Increase you chances, reduce your odds etc. I used to back 2 horses in a race for a long time but now I [mostly] just back 1 horse per race.