Also, are you guys coming straight to practice from work? Does she have a job she hates? Has everyone had a meal or a snack before practice? These are factors. I was in a band where we kept granola bars on hand for this reason because we practiced at 7 p.m., everyone was coming in rumpled and grumpy with low blood sugar and our rhythm section would straight up BEEF. You cannot come to practice that rundown and expect anything good to come of it.
OK, so once you talk to her and get her to be thinking about how she is communicating, go to the other two and tell them that you are not blind to the fact that your singer can be painfully tough -- but they need to stop insulting her because there is no room for that in this band, that it's inhibiting to creativity and obviously just makes practice awful. It's only making things worse. Tell them that you have learned not to take it personally, that she is just "intense" or a "perfectionist" and that you know she is working on trying to chill out and make your practices a bit more peaceful--and that you need them to do the same, to reel it in and toughen up a bit. Ask them how they feel things are going otherwise, hear them out, see if there are ways to placate their hurt feelings. You may need to referee a band meeting and clear the air if this doesn't change anything.
Beyond that, I think you might have to occasionally just step up and intercede. If they start going at it, leave the room for a few minutes, or address things immediately as it happens when they start acting like children. No one likes being told what to do, and being in a creative collaboration only makes it worse because you are already vulnerable. I remember hating my best friends after practices when someone got bossy or when someone refused to take criticism. It's kind of the nature of being in a band. If someone does actually honor their threat to quit, maybe you should consider it a blessing and find someone who is more agreeable and a better fit with the group dynamic.
Best of luck, Fan
See also: -Crotching Whiskey at the Justin Bieber Concert and Getting Thrown Out: A Review -The 15 Most Ridiculous Band Promo Photos Ever -The Ten Worst Music Tattoos Ever
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