How do I do stop my 3 month old from sucking her thumb?

Personally, I think it's okay for a baby that young to suck her thumb. It's a reflex action and really natural. There are some advantages to a thumb sucking baby. With her thumb, your newborn has a built-in natural way to calm herself down any time she’s feeling cranky. If you're really uncomfortable with baby sucking her thumb, you can use a pacifier. I know my late grandmother had really old-school methods to stop me and my siblings from thumb sucking such as putting a spot of lemon juice on the thumbs but I wouldn't do that to my kids personally. As your child grows, too much thumb sucking may disturb the alignment of your child’s teeth and even the structure of her mouth, so you’ll definitely want to wean her off before her permanent front teeth come in, around age six. For now, I think it's okay :)