Putting the third line into a tank is dead easy. All the hard work is in putting a vent into the tank so that it opens at the very top of the tank, and you gotta do that no matter what kind of tank you're putting together. So adding the third line is just sticking a short tube into the third hole. Of course, you usually have to open that hole but that's dead easy too.

I used to run uniflow in everything I flew, even fuel RC. But it pretty much requires you to use either the pickup line or the pressure line as your fuel input line and you have to open up your vent line while fueling as well. And you then have to plug the vent line and reattach your fueling line (unless you've plumbed that line with a T and fuel dot). I'm not flying competition any more and have discovered that I really don't need uniflow on anything I fly eventhough I FLY them suckers HARD. So I make tanks that're very simple to use at the field and lot's easier to stick together than uniflows are.

Three tubes: One vent that curves to the highest point in the tank. One line that just sticks through the stopper. One line that sticks through the stopper and gets the clunk line.

Plug the line from the carb to the clunk line tube.
Run a fill line to the short tube that just sticks through the stopper. This line is plugged except when it's used to fuel the tank. I use a 1/4" 6-32 bolt with a large head that's easy to grasp. My lines barely stick through the fuselage side or cowling bottom or on the Sukhoi, just hang there in that big old open cavern.
The muffler pressure line runs to the tube that runs to the very top of the inside of the tank. When I crank my fuel pump, the tank fills until it's full and then starts to spit overflow out the muffler. I give the crank half a turn backwards to stop any siphoning that might be going to the carb, pull the fueler and stopper the line into the airplane.

I also count how many cranks it takes to fill the tank. It's a habit I picked up flying competition CL aerobatics. It does a number of wonderful things. It proabably is going to be a problem for you guys flying the big birds. It tells me when an empty tank is about full and on later flights how much fuel I had left from the previous flight. It also helps minimize the amount of fuel spit into the muffler.

When I first started flying this simple setup, I really wanted to see just how bad it was going to be compared to my Uniflows. I flew the test airplane with it's old, perfect feeding uniflow and immediately swapped in the new tank. Set the needle and flew. Outside loops, long inverted flight, squares inside and outside, snaps right side up and upside down, and flew until it went dry in the air. I couldn't see or hear enough difference that it mattered.

Only having to uncork one line, and that being the one the fuel probe is going into, is nice and easy. And no extra plumbing is even easier. I've never bothered to buy a fuel dot and chuckle ever time I'm in the LHS looking at all that plumbing stuff they want all that money for.