How to delete all the emails automatically after 3 months, it should go into the delete folder after deletion automatically?
You'll need to specify a retention label:
To assign a retention label or archive policy to a message or folder:
In the message list or the folder pane, right-click the message or folder that you want to assign a policy to, then select Assign policy.
Here's an example of the types of retention labels and archive policies that might be available:
Select the retention label or archive policy you want to assign to the message or folder.
Messages removed by a retention label might be permanently deleted, or they might be moved to your Deleted Items folder. How long the items are recoverable is managed by your organization’s IT administrator. The default period is 30 days.