How to get three and a half year old to stop pooping in her panties?

She starts preschool in august and can't go if she keeps doing this. We've told her and reminded her and asked her if she needs to go but she never goes in the potty. She pees great but poops in her panties, sometimes three times a day. She also has a pacifier still. Any advice on that? She cries and cries for it until we give in. Ugh help.