I have not seen my nephew and two nieces for 3 years now. I miss them badly. They were taken from my sister. I want to know where they are. How can I find them?

First its not a good idea to piggyback your question on someone else's. This can lead to confusion. You should start a new thread. So I've moved your question to its own thread.

You need to find the caseworker that handled the case. And see if you can be put in contact with them. Depending on their ages when put in the system and whether they were adopted or not, will be factors in letting you have contact.