If you could see some of the crazies that come to adoptions, you'd be amazed and more understanding of why rescues are so strict. I occasionally foster dogs for a couple of local rescues, and the stories I've heard will curl your hair. The latest was a lady who wanted to adopt a dog, but couldn't touch it (allergies?). Seriously? Another person said they don't give their older dog heart worm preventative anymore b/c "mosquitoes don't like old blood, right?" (seriously, someone actually said this!!). Other potential adopters won't agree to the spay/neuter clause b/c "they'll watch the dogs to make sure nothing happens" - right... Other people want a "yard dog" - hmmm, sorry, but why have a dog if you're only going to throw it out in the yard and ignore it?
The rescues don't want their dogs going into yet another undesirable home. The dogs have already gone through a lot in many cases, and they don't need any more bad experiences. Dogs that are in foster homes have been evaluated, and their special needs are known. If the rescue doesn't think the dog would be a good fit, there's probably a good reason for it. The dog may be an alpha dog and need an only dog home, it may need to be placed with a dog of the opposite gender to avoid same-sex problems (common), it may need to be placed in a home where the people are always there (separation anxiety), it may be an escape artist and need a tall privacy fence (some dogs can and will climb chain link - been there, done that), etc., etc.
To the OP - wanting another female to add to a home already having 2 other females raised a red flag for me. There are many rescues who won't adopt female/female. Sometimes it works out fine, but when it doesn't, it gets UGLY fast! Females often vie for dominance in a way that's much more vicious than with neutured males.
I agree that many rescue people are seemingly over-zealous, but when you've repeatedly seen the abuse of dogs that they have (rescuing from shelters and abusive/neglectful situations), it makes their position, IMHO, more understandable.