Wyatt,
I don't understand the problem. If you have enough Hobies in your area to have a successful regatta with Hobie-only (and you probably do), why would you alter that just to accommodate three non-Hobie boats?

Are you saying that the other Hobies from other fleets in the area will not come to your regatta if you have an open class? If that is the case, I would say it is a no-brainer -- just don't have an open class.