@Sarat Chandra - The community expert label is specific to tag. We must have 3 accepted answers in last 30 days from a tag to gain this. And it apples at the time of answering and accepting the answers. You can find more details in https://learn.microsoft.com/en-us/answers/support/community-experts (following in an important point to note about community expert label)
This feature takes into account the snapshot of states and data at the time the answer action is taken. Thus, we don’t compare the previous or the upcoming tag changes to add this label. For example, UserA is Expert in TagA and they answer a question from TagB. Later re-tagging that question with TagA won't add the Community expert label in that question.