In order to have different assignment emails, you need to create two different approval processes with different entrance criteria.
The first process has criteria status != rejected and starts at step 1.
The second process has status = rejected, and starts at step 3.
Another option would be to instead try to use skip criteria on approval step 1 & 2, which should be feasible, but you would get the same assignment email.
I imagine you could use a visualforce email template that could render differently based on status, but have not tried this on an approval process before.
It's likely going to be easier to maintain just one approval process, so might be worth testing the second solution first. I'm pretty sure it would work but haven't done exactly that before.
One note, if you want to go back to the exact step it was rejected at, you should be able to use a field update on rejection to log the step it was at. Then use skip logic to determine which step to start at. It wouldn't be practical to create a separate approval process for each possible step. If you elaborate on your needs I can think over it further.