I am not sure I follow you in your description, so let me say this.
Simple answer is, all flow is flowing through the OUT port as that is the path of least resistance .
Before you added the Post pounder, you had a hose from the hyd blk PB hose going to the back for the 3pt.
Remove this hose from the 3pt in port.
You should have inserted the Post Pounder in series using the PB from the hyd blk going to the Post Pounder valve IN port, and the Post Pounder PB hose to the 3pt IN port.
Did you purchase the PB adapter for the PB port? If you did not, the flow is flowing through the OUT port to tank. No 3pt and no Post Pounder.
Although that port is a PB port, it can be used for other things.
You need the PB adapter, which separates the the pump flow from the cyl flow.
There are three fittings that can fit that port.
Open center plug. Allows pump fluid to flow to the OUT port.
Closed center plug. Blocks fluid flow to a standby state.
PB sleeve. Allows pump flow and blocks cyl flow.
It may look like this.