When soldering 3-D pieces together, first tack the panels together with a single tack at each end. If it later turns out that there is an alignment problem, it is much easier to dis-assemble a few tacks, with a piece of paper inserted into the space between the pieces of glass and moved up into the molten solder while your iron is at the tack joint. The paper will strong enough to move through the solder, separating the two piece of glass.
Once your 3-D piece is tacked together and looks OK, turn the piece over on its side, and, using 50/50 or 60/40 solder, fill in the inner seams, moving the piece around. Be careful to support the piece with boxes or blocks and by holding it at the top part above where you are soldering, to prevent the piece collapsing.
Once the inside of the piece, say a panel lamp, has been soldered smoothly with 50/50, turn the lamp over. Get a few boxes or similar supports to prop the lamp up against, and make it so that there will be a level solder seam. Using the 50/50 solder again, fill in the seam. It doesn't have to be perfect, at first. Do all of the seam filling first, to ensure the stability of the piece. Then go back with 60/40 solder and, again making sure the lamp seams are level, finish by smoothly soldering each seam.