RE: "Purchase Order Workflow Error"
Hello Saqib,
could you share the workflow setup with us? Is it the standard one?
Some ideas so far:
a) If the 3 approvers needs to really approve, I guess you are playing with 2 "An approval request is approved" Events, one filtering by "Pending Approvals: 0" an the other "Pending Approvals: >0>, right?
b) Inside the "Pending Approvals: 0" you should have a Response to "Remove Record Restriction", and this should happen only when "Pending Approvals: 0" which means that the 3 approvers already approved the request.

If this does not help, please also check in the "Pending Approvals: >0" event, the response "Send approval request for the record and create a notification."

This should point out to the initial "Send approval request for the record and create a notification." in the Next Then:

With this, it should work as expected, otherwise please provide us with more details so we can try to understand your problem better and help you out.
I hope it helps