What is the correct number of stereo isomers in case of 3,6-dimethylpiperazine-2,5-dione?
Three is the correct answer.

As Ivan pointed out, there are no planes of symmetry, for example:



Ivan is also correct - the "RS" is the same as the "SR". You can prove this to yourself most easily using molecular models, but here is how to see it on the page. You just need to rotate the molecule by 180° in the plane of the page:



So, despite having chiral centres, this compound is achiral because it is superposable with its mirror image. "But wait, I was told that a molecule needs to have an internal plane of symmetry to be a meso compound" I hear you say. Unfortunately, this is an incomplete truth that many sources repeat. To be a meso compound, it can alternatively have a centre of symmetry, also known as an inversion centre. This is simply one point in space through which any atom is reflected through - ie. each atom is diametrically opposite an equivalent atom at an equal distance from this point.



The other two stereoisomers are the (R,R) and the (S,S). These are non-superposable mirror images, and hence enantiomers. Note these isomers have no plane of symmetry or centre of symmetry.



Once again, this is easier to see with physical models.