First of all in no way is she behind at all. Don't even let that thought enter your head. Based on what you are writing, I would say you should give her more time and help her get ready by getting her a little potty of her own, letting her sit on the potty, reading books about it (Everyone poops, etc), keep showing her when you need to go, asking if she would like to try to use the potty, keeping positive even if she says "no" or doesn't do anything when she does sit on it, giving her incentives for trying, celebrate success (but not so much that is creates too much pressure), and eventually going out with her to buy her choice of undies.
As you know of course, the age of potty training varies; some children can be 4 before trained. Also, the degree of potty training varies as well - you will hear parents saying their 2 year old is potty trained, but really the one who is trained is the parent because the parent is making sure the kid gets it done on the potty rather than letting the child really own/control their own potty experience. I never really understood the rush rather than letting the child transition to the potty in his/her own time as s/he discovers his/her body, control and pursues independence.
Before considering starting potty training, I would recommend to be honest with yourself that you TRULY see signs of readiness.You will know (and the fact that you are posing this question means you are not sure she is ready - trust your instinct; you're right that she doesn't sound ready). The more of the below signs of readiness she displacy the more likely your child will make a smooth transition to using the potty.
1. Regular and predictable bowel movements
2. Indicates through words, grimaces, posture that she is urinating or making a bowel movement
3. complains or appears uncomfortable when diaper is wet/soiled
4. begins to pull pants down
5. asks to use toilet or a child-potty
6. asks to wear underpants
7. wants to please/can follow instructions
8. stays dry much of the day
9. shows an interest in handwashing and keeping clean
Hope this helps. I found that the less pressure the better the process. :)