Good to see the tangential pond returns for the bottom drain.
You're going to have a hard time priming the pump when starting. When the pump is turned on the water in the filter is going to draw down and that pipe out of the filter is probably going to go dry. Moving that pipe down might fix that, but that depends on a lot of things.
For that size pond I'd use a submerged pump rather than an external. Easier, cheaper.
All evaporation will show up in the filter. Meaning you might lose 3", 12" (don't know the filter size) of water per day in that filter. Putting an auto fill in there can fix that problem.
The 3 stage filter deal is a pretty old design. Much better filters designs are available today. Self cleaning, cheaper to build and 10 to 30 times more efficient. Google, sieve filter, trickle tower, shower filter, moving bed filter.
You might consider using your pond as a settlement tank like this:
The result is the same if debris sits in a filter or on the pond bottom.