The middle pipe is easy, we just measure the distance between the front and entry pipes, subtract the length of the middle pipe, divide by two, and mark that distance from the two pipes already installed, mark lines, and inlet just as we did the front pipe.
With all three pipes inlet, we can mark locations for pins. We square a line from the front of each pipe around to the side. Then we mark a line 3/4"(.750) back from this. 3/4" in this case, because the pipe is 1-1/2" long. We put a mark on this line (I use a circle) so that we know this is a drill location. Marking designations like this on your lines is a good habit to get into as you will find out the first time you drill a hole on that "other" line.
These lines locate the hole locations along the length of the barrel, but not in relation to the top or bottom flat of the barrel. I'm going to talk a little extra about this, because of the interesting topic on pinning that has been up. The pin holes for my barrel lugs and the pin holes for mt pipes are not at the same distance from the bottom of the barrel.
I want the lug pins as high as possible, and since the lugs that I use don't have side tabs, I can take advantage of the depth of the dovetail, so that the drill would rub against the barrel bottom if the dovetail wasn't there. I am using all 5/64"(.078) pins so the drill's center is .039" below the bottom of the barrel.
For the pipe tabs, I want the pin to go through the middle of the web. My web is .200" thick, so the drill point is .100 below the bottom of the barrel.
I drill my pin holes in a drill press. By placing the top of the barrel against the non moveable jaw, we control where the bottom of the barrel is all the time, even with or without the stock. But how do we accurately locate .100" below the bottom of the barrel?
One way to do it is to take the barrel (or in my case a piece of 1" barrel), tightening it in the vise with the bottom flat blackened (thank you Sharpie) and putting a Number 7 drill (.201") in the chuck. We adjust the vise until the .201" drill rubs, and then we know that the axis of the chuck is very close to .100" from the bottom flat.
We switch the .201" drill for a 5/64" drill, align our marked layout lines with the drill point and drill with confidence.