The same company has applied for three positions (the company informed me to go for an interview, but didn't say which position, what should I do? How to talk to HR in the interview tomorrow?

Of these three positions, which one do you think you have the most advantage? Take this most advantageous position as the position you are applying for, and talk to HR. Generally, large companies will ask you to fill out an information form when you go for an interview. If you fill in the position of your greatest advantage in this form, there should be no problem. I wish you success!