First issue is that you need some serious flux. If you don't already
have some, go to a welders or plumbers supply shop and get some of
the paste flux used for brazing copper pipe. I use stay-silv and it
works great. Second issue is that copper is a great conductor of
heat, so you will need a big, big flame if the piece has a large
thermal mass. Copper oxidizes so quickly and heavily that it will
exhaust even heavy-duty flux fairly rapidly, so you want to get the
piece up to temperature and the solder flowing without undue delay.
The third issue is, how does one make a four-sided triangle? Are
you speaking of a four-sided pyramid? Triangles are two-dimensional
and have three sides only.
Dos Manos Jewelry