Abusive people commonly use this tactic to get you to agree to their demands. She is not actually going to kill herself. (If I'm wrong, then good riddance to her. I don't want her in the human gene pool.) As others have stated, what you should do is leave this relationship and forget about her.

Looks like your committed

If you have met her parents or have their phone number, call them and tell them what she said and explain that you obviously can't keep dating her but that you hope they'll get her the help she needs. They've probably been dealing with her issues most of her life. You've dated her for 3 weeks. You don't have any other responsibility to her unless you feel that you love her.

What did u pick?

Thats psycho, don't do it! Drama this soon in the relationship, will mean drama for life.....

Let her.

Okay if you don't want to marry her blindfold her insist its for a nice date or something special if you don't want to lie take her to a mental facility (something may be wrong with her) inform them of the situation before you go inside tell her you're not gonna marry her if she freaks out you have help for her right there if its a bluff... Well you got her good

Let her kill herself, you don't want to be chained to a psycho.

People that use that threat, won't kill themselves, they just do it, they don't announce it like that, say fine and walk away

Do whatever you can, but if she refuses help then there's nothin you can do after you dump her crazy ass. Not your problem.