I have read posts where people use a header tank along with the regular tank. What is the purpose of this arrangement and how is it set up?

Its a bubble trap, in case the clunk in the main tank comes out of the fuel during violent flying, we use them on helicopters all the time.

It goes, muffler pressure line to main fuel tank. Main fuel tank clunk line to header tank pressure nipple (at the top of the tank) then a clunk in the header tank feeds the engine.

what if the header tank has two vent lines, what can you do to the 3rd to make it work?

They were used in Ducted Fans a lot to give better fuel levels for the very large bore carbies.

Plug it up. In aircraft today you should not need a header tank, I can't see where you would need one. Set the fuel tank and fuel lines up properly and you would never need to use one.

Cheers

Some planes are set up with the tank on the CG. Depending on the size of the model, that can be pretty far from the engine. With some engines, that matters. Having a secondary tank near the engine cures that problem.

In fact, guys discovered years ago that a large fuel filter close to the engine helps solve the same problems.

Lot's of things work for specific problems and aren't needed everywhere.

Ever hear of a "chicken hopper" tank?

You have already been informed on the set-up. OTOH, the reply, "Set the fuel tank and fuel lines up properly and you would never need to use one." I find that reply a bit restrictive when using "Never".

I use Header or "chicken hopper" as they may be termed fairly frequently to assure good fuel flow.
The #1 time is when a fuel tank is tight inside a small fuselage area without adequate padding. It is easy to get fuel foaming which wrecks havoc on good operation. Another time is when in a rather large model, the tank is not in a really good position to assure good flow, especially to some of the glow engines that require significant pressure to maintain the Needle Valve setting, those engines that have overlarge Carb. ventures.

I have a SPAD with a gutter pipe fuse. that is fun to fly. It is the standard .40 size, but I use a .60. I could only fit an unpadded 6 oz. tank in the fuse. so I use a 2 oz. header outside with 1/2" rubber pad. Works great. More fuel and the foaming doesn't get to the header. I also used a header in a large model with a Webra 1.20. Unless one is using a bladder tank, I find the header an effective alternate based on the situation.

On my SPAD Canard the egine is mounted at the back of the plane and uses a pusher prop. The fuel tank is in front of the main wing which keeps it about 12 inches in front of the engine. Fuel flow to the engine was erratic so I added a small 2 oz header tank close to the engine.

On my SPAD Flying Lawnmower the main fuel tank is mounted inside the wing and at 90 degrees to the fuselage and about 14 inches away from the engine. I use a 2 oz header tank near the engine to keep the engine happy. Never a fuel problem.

Fuel flow is as stated above.