You may need the long style test balloon to reach down the cleanout and into the main line. Basically, you inflate the balloon to 25psi or so and fill your system up. There is a metal ring attached to the balloon, so the balloon won't wash away if it deflates/slips. Once the test is done, slowly release the air from the balloon and it will let the water flow past and into the septic. You may have some water splash out of the cleanout when the air is released.
You are a bit lucky that you only need to test with 3' of head as many places require 10' of head.
I do have a long 4", a regular 4", and a 2" balloon from my test (only used once). If the PMs would work on here, we could arrange something (4" regular or 4" long). I don't need them anymore, so they might as well go to someone who can.