By her do you mean Elish the younger female? The youngest 3 are all around the same age so can keep each other entertained and give poor Crackers some peace!
There were only three giraffes on-show when I went, I know that the two smaller ones were the new giraffes, but couldn't get a close enough look at the taller one to identify her. (There wasn't one in the giraffe house either, before you ask.) I'm not sure whether or not its a permanent situation, but will check again next time I go.
I have no idea, but there were still two giraffes when I visited just a couple of weeks before. It is possible that she was off display for whatever reason, but this is pure speculation. Sorry I can't be of any help.
Off show where? There's not really anywhere you can hide a giraffe at London.....if you went inside the house as well and only saw three, then there were three.