[indian]

configuration t2g 3 eg 0 doesn't go  under distortion . What is the reason behind it??what does degeneracy actually refer to??

[Rudi]

Three electrons in the t2g orbitals is an unambiguous situation, because every oribtal is occupied by one electron. Thus, there is no need for a distortion of the system.

What you mean is incidentally not called "John tellar" but Jahn-Teller distortion.