You cannot un-ask the bridal party without causing hurt feelings. If you're telling them ALL that you have decided to no longer have a bridal party, then go ahead and do it (and be sure to refund them for every penny they may have spent on your wedding), but don't be surprised if they are hurt and angry by your poor planning.
No, you cannot make up some lame fake title to try and hide the fact that you're being rude. It's unnecessary, rude, dramatic, and stupid.
WTF is the difference between "Bridesmaids" and "Best Girls"? That's just obviously pointing out to them that you no longer want them as bridesmaids. I mean, "Our plans changed and we're going to scrap the whole bridal party" is one thing ... but "Our plans changed and you're no longer bridesmaids, but I made up this phony lame title for you and you still need to dress in the color I pick for you, even though you're not bridesmaids" is REALLY insulting. It's basically saying that you want them to do everything a bridesmaid does (because a bridesmaid's only official duty is to get the dress and be in the ceremony), but you are denying them the title. That'll just make them think, "Geez, if we're already buying specific dresses to be 'included,' then why aren't we real bridesmaids?"
And three bridesmaids is not very many at all. Not to mention that I don't know where you got the idea that three bridesmaids constitutes a "proper grand princess wedding" when most of the REAL princesses out there (the Duchess of Cambridge, Crown Princess Mette-Marit, Princess Stephanie of Luxembourg, Princess Madeleine and Crown Princess Victoria) had only two, one, or zero bridesmaids in their royal weddings.
I don't really get how kicking three friends out of your wedding is really going to make that much of a difference. You can reflect the informality of the new location by picking more informal dresses for them - or, like you said, just pick a color and let them get their own dresses. Do a Google Image search for "mismatched bridesmaids" or "different dresses same color" and you will see a ton of nice examples.
Either suck it up and leave the bridal party how it is; or apologize profusely to the whole bridal party and tell them that you're scrapping the bridal party since your plans changed. But DO NOT try to come up with some kind of consolation prize for them. Just scrap the bridal party or don't ... but if you go with some bogus in-between solution then you're going to piss off a lot of people and look like a real fool in the process. Make ONE decision.