I think everyone would do it a little different then the next guy. Since it's only 3 TPR's, I would do it like this.
I would install them about 1 1/2' off of the bottom. This will help a lot to move any waste that will settle. I also think a must for your pond is a diffuser drain. A diffuser drain will also hope move the waste to the drain.
Another big thing will be slopping the pond right. The end without the drain will need to be slopped good to the drain.
What kind of flow rate are you talking about on the TPR's?
That's my 2 cents.
Tom