will the case from my apple watch 2 fit the 3 ?

Hi

The back crystal on Apple Watch Series 3 is 0.25mm deeper (equivalent to the depth of 2 sheets of paper) than on Apple Watch Series 2.

Otherwise, the dimensions for Apple Watch Series 3 are unchanged from Apple Watch Series 2.

0.25mm is a very small difference.

If in doubt, I suggest that you contact the case manufacturer and ask them.