Good Morning, S.!
First off, there is another mom wanting similar "thumb sucking" advie this morning, so you may want to read her responses, as well as yours.
Your daughter seems so young to worry about letting go of her thumb, although the "teeth thing" is probably your biggest concern. I was worried about this with my youngest son and his pacifier. He loved that pacifier for a long time, and I saw it affecting his teeth. I eventually put his "needs" over his teeth, knowing that the baby teeth would fall out and we would have another chance...maybe. His teeth haven't fallen out yet, but they will soon enough, he's 5. I'm not sure I would worry about your daughter until she's older. She's still pretty young, and her age seems normal to still suck her thumb. But she is also at an age where you can "minimize" the sucking, by gently asking her to go to her room when she needs to suck her thumb because it means she's tired and needs to lay down. She'll eventually get too "busy" to want to go to her room, and this will begin to minimize her thumb sucking. She's still very young for this, but it will be a good start, I think. There are going to be those times in the car, family trips, etc...that you will have to bend the rules. But when you're at home you can control the issue more.
The thing I wouldn't do, is to make her feel so aware of her thumb-sucking that she becomes insecure about it. Your home is the only place in the world she can be totally herself, and you don't want to "shame" the issue, so she begins to hide it at home. If you can avoid doing that, minimize the sucking, AND still have a confident little girl...then you would've performed a mini miracle :o)
Good Luck
:o) N.