Unless she is ready for the potty it could easily backfire and make training much more difficult. Age and interest don't necessarily indicate readiness. Does she stay dry for more than 3 hours at a time (like during a nap), can she tell you if she's wet or dirty, does she dislike being wet or dirty? Language skills are also important, so she can talk and understand what is going on.
Instead of camping out in the bathroom, I would suggest buying a package or two of the cloth training underwear - she can feel if she's wet but they are easier for cleanup than regular undies. Talk up how great it will be to be a big girl and go on the potty and let her pick some pretty panties for when she's done. Then, this may sound gross, but let her see that you go to the potty (mom only is great, she doesn't need to see dad do it) so she knows it's a normal thing that everyone does. Just leave the door open a bit and tell her "mommy needs to go potty. Do you want to try going potty too?" Modeling is the best way for kids to learn.
You may also want to talk to the preschool. Not all schools require kids be 100% trained at 3, especially if it is only a 3 hour program. And many kids who are 'trained' will have accidents once or twice at school, even in to Kindergarten, so they should at least be aware and prepared for that kids of thing. She can wear pull-ups and carry an extra on in her backpack. Again, seeing other kids go potty may be all it takes.