It would take me some time to adjust to the idea; because even though I've always considered the possibility that one of the three of them could one day announce that, I've also done the usual of thing of taking for granted that they are not.
My big thing would be that I'd want them to know it made no difference to me. I absolutely adore all three of them (now grown) for the people they are, and have always been. It's that simple.
I'd hope, too, though, that they'd understand that I would have a little bit of "getting-used-to" to do, and a little bit of re-thinking some of the things I may have been thinking (like imagining them marrying someone of the opposite sex and coming by any family they wanted the "usual" way). So, I'd hope they overlooked some things I may say or do out of not being completely settled into "the new news" I'd gotten.
I'm not sure someone who hasn't had his own children, or even someone who hasn't reached the stage where they've grown up, can imagine how much a most mothers love their grown children (at least the ones who have always loved their children in the way that a mother "is supposed to").
Thinking about each of my own three grown children, and imagining what I'd do; it's so "without-a-doubt" that I can say it wouldn't make any difference in how I feel about any of them.