Well, she shouldn't feel obliged to keep her child just to educate the adoption agency!
But there's absolutely no reason at all for her to feel that she can't be a good mother because she's a lesbian. Many, many people - whether because they're lesbians or single parents for other reasons - do a wonderful job bringing up children without having a mother and a father present.
Extensive research has shown that children of lesbian or gay parents are just as emotionally healthy as children of heterosexual parents. They are also just as likely to be heterosexual as anyone else, in case that's what she's concerned about.
I'd suggest that she do a search on Google or Amazon.com on "lesbian parenting" - there are plenty of books and resources out there.
If she feels she simply can't cope with being a parent, that's one thing. But being a lesbian doesn't in any way mean that.
As an additional note, a 3-year-old being placed for adoption would be considered "hard-to-place" in many areas. Most of the heterosexual couples looking to adopt are looking for healthy newborn infants, preferably white. Older children are seen as "damaged goods".
So if she did relinquish her child for adoption, the likelihood is that it would spend considerable time in foster care, and might never be adopted at all. And if it did find adoptive parents, it might well be placed with a single parent or a gay or lesbian couple.