by emptyglass » Sun Sep 16, 2012 7:14 pm
I can suggest a method, but it isnt the true geometrical approach your after.
First, mitre two pieces, as per a normal mitre,
Second, take a holesaw and notch the 3rd piece in from both angles, same as the mitre, but only half way through the 3rd tube. File or grind to finnish.
I've used this method on ute trays with 3" pipe and it looks neat, but there is usually one "long" side, so it makes the joint look nice. But I don't think it will be what youre looking for.
You could do a trial and error approach, if you could use a mitre or drop saw. use some cheap mild steel for the mock up, keep adjusting and cutting and mocking up till you get the angle, then make a template off the sample once you have it right.
Good luck.