combining registration/login/logout in subdomain sites

Hi - I have three sites and
Each is a separate install of the latest WP.

None of the sites currently have members except for Admin.

I want visitors to be able to register at any one of these sites and then login from any one of them. Likewise when they log out they should be logged out from all of them.

In other words I want the membership to function as though it was one site.

What is the best/easiest way to do this?

For instance - would it be easier if the three sites were part of a multisite installation? And if so how could I set this up as they werent created this way?

WOuld it be even better if they were part of a Buddypress install so that I could use some BP plugin to combine the 3? (I dont really want to use BP but if its the easiest way... )

Or could a developer somehow use cookies that were accepted by all three databases as proof of being logged in ?

I hope Im being clear enough here - Im a bit lost in the possibilities but feel others must have done this before?

Thanks - DV