DD is 3.6 and still has one for naps and bedtime. I dont have a problem with them. (Obviously they will go before next teeth come in, but the "damage" to these teeth is done already iyswim)
We did it gradually. at around 20 months she was only allowed her dummy for rest times. So fine, if she wants to sit and watch tv or just lie on the sofa/bed/carseat/pushchair. NOT FINE to walk around/talk with it in. NOT FINE AT ALL. And it would be removed from her completely if she broke the rules. Harsh at first, but she soon learnt that rule and would get it, sit quietly for 5 mintues, then put it down and go and play etc.
By her second birthday (or just after) she was in a proper bed now a cot. The dummies stayed in bed and if she wanted them, at ANY time of day, she could go to them, but no-one was going to come with her, or make her dummy time more interesting. We did still take them out with us when I knew she was liekly to need them for nap time eg in the car/pushchair.
just after her third birthday we bought a lovely sparkly pink box and when she gets up in the morning she puts them in the box and I decide when she can have them (eg not to rest, but yes at naptime) because she is getting to the point of not napping, or wantin a nap at say 4pm, and a suck of the dummy would send her straight to sleep, and i didnt WANT her to sleep at 4pm!
She knows that one day the dummy fairies will come and take away her dummies, I am thinking of doing this just before our summer holiday, perhaps a couple of days before hand, then the new place will help her lose association (desperate hope) - if however we are not brave enough for this route, I am sure that school will shame her out of it 