Katie, you sound so much like me.
I hate to offend and so can walk right into trouble knowing full well what I'm doing but just so that I avoid hurting feelings.
Which is why I have learned to find subtle ways to deal with that: I turn the "negative" I wanna say/do into a "positive". Like someone else suggested, find out the names of the stylists. Then call and ask to speak to one of the good ones (Do Eenie Meenie... if you can't decide between the two) and then tell HER (the good one you speak to) that you have heard such good things about her and that you INSIST that SHE be the one to do your hair. Since I imagine (like me) you wouldn't wanna say straight up that #3 sucks at doing hair (although you and I really do need to grow some balls and learn to speak our minds
) Then just tell her that you really want no one else but her to do your hair coz you heard good stuff (from your sis who witnessed her talent from the sidelines during her visit -- if she asks your source). Tell her you are so serious you will reschedule your appointment if you have to, so SHE can be the one to do your hair. She'll probably be so honored that she'll not want to share you with anyone else.
And then if you like her work, you can always ask for her. No one will take offense thereafter if you do. Coz humans are just finicky like that when it comes to getting service; we like consistency, especially if the service we got last time was A1.
It's interesting that the one with bad hair is the worst at doing hair there. You'd think the one with bad hair would be the good one coz you'd assume she does the hairs of those with good-looking hair. Hmmm...I guess not