Is it right

Big O  Worst case

Big Theta  Averge Case

Big Omega  Best Case
Is it right
Big O  Worst case
Big Theta  Averge Case
Big Omega  Best Case
Not quite. All three notations can be used for bestcase, worstcase, or for averagecase as these notations give asymptotic bounds on a function that may correspond to the bestcase, the worstcase, or the averagecase running time. These notations may often be used in the sense you mention, but they donâ€™t have to be. Letâ€™s take an example.
The best case running time of insertion sort is linear. A linear polynomial is BigO(n) but it is also BigOmega (n), and therefore it is also BigTheta(n). So it would be perfectly legitimate to say â€śThe bestcase running time of insertion sort is BigO(n)â€ť. We donâ€™t typically use the BigO notation with the best case because thatâ€™s not very useful for us. For the best case, a BigOmega lower bound is more useful for expressing that we canâ€™t do better than this. For the worst case, BigO is useful because it tells us we canâ€™t do worse than this.
When using these notations to express a fact about a running time (best, worse, average), think about whether you want to say bounded from above (BigO), bounded from below (BigOmega), or bounded from both above and below (BigTheta).