They are all particularly strong fighters against the Black Thing who lost their lives in the battle. As Mrs. Whatsit, Mrs. Who, and Mrs. Which, they are something between ghosts and guardian angels.
Although they can advise current fighters (Meg, Calvin and Charles Wallace), they can't fight themselves anymore and they are losing touch with the human world over time. Mrs. Whatsit, whose lost battle was the most recent, can easily converse. Mrs. Who communicates most easily in quotations from other fighters for the light (the Bible, Pascal, Cervantes). Mrs. Which can barely speak at all. It isn't clear whether they are degrading into nothingness, or if they are moving from a human to a divine realm. Mrs. Which, the oldest and strangest, is also the most powerful, which supports the second theory.
As described in the following passage, Mrs. Whatsit was once a star. One could guess that when Mrs. Which says "It was not so long ago for you, was it?", she is implying that Mrs. Who and Mrs. Which were also stars who lost their lives in the fight against the Black Thing. It's not conclusive though.
"It was a star," Mrs. Whatsit said sadly. "A star giving
up its life in battle with the Thing. It won, oh, yes, my
children, it won. But it lost its life in the winning.
Mrs. Which spoke again. Her voice sounded tired, and
they knew that speaking was a tremendous effort for her.
"Itt wass nnott sso llongg aggo fforr yyou, wwass itt?" she
Mrs. Whatsit shook her head.
Charles Wallace went up to Mrs. Whatsit. "I see. Now
I understand. You were a star, once, weren't you?"
Mrs. Whatsit covered her face with her hands as though
she were embarrassed, and nodded.
"And you did-you did what that star just did?"
With her face still covered, Mrs. Whatsit nodded again.
Charles Wallace looked at her, very solemnly. "I should
like to kiss you."