If the septic tank needs upgrading on a 3 year old house, I would be extremely wary of buying it without getting my own engineer to give everything the once over. If there is an engineer's report confirming that the problem is fixable and the cost of doing so, most lenders would probably let you go ahead, but would require the work to be done and would have a retention on the cost of the work until such time as it is done and passed by your engineer.
Far better to get this work done before going ahead. If it is built into the price that you are getting a sub-standard septic tank, far better to agree a higher price on the basis that buyers must pay your chosen contractor to come in and fix it. One way or another, you will hardly be able to continue with the sub-standard septic tank once there is a lender involved.