Hi @HASSAN BIN NASIR DAR ,
For detailed explanations on the 3 types of retention tags, hopefully you can find the table below helpful:

If I will create all 3 tags with different names but when I will create a retention policy and add all 3 tags in the same policy. In that case what tag will be applied?
As described in the table above, #1. DPT (Default policy tag) would be applied to all untagged items in the mailbox; #2. RPT (Retention policy tag) would be applied to default folders like Inbox, Deleted Items, etc. in the mailbox; #3. Personal tag would not be applied to any item unless the user manually assigns a personal tag to items or folders in the mailbox.
Second question is: Is it best practice to create this kind of tags and add them into same policy?
You can definitely create a retention policy with all the 3 types of retention tags as they don't conflict with each other.
See Create a retention policy for Exchange Online.
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