Sharing a folder with specific users/group

I am trying to make 3 different shared folders for 3 different groups on Windows server 2008 R2. I have tried creating 3 folders in C:\, and then share it with the specific groups. The problem I have is that even though I have only given read and write permissions to the specific group members, the other group members(from the 2 other groups) can still read and write in that folder. Which is not supposed to happen. What am I doing wrong and how can I fix this?

PS: I am quite new to this...