By default, a tab character tabs to the next column whose number is a multiple of 8. By default, input lines too long to fit on a single output line are wrapped, and a double vertical bar is put in the left-hand margin to indicate that this has happened. This can be helpful for debugging if your printer is connected to a monitor, but it can also be really annoying if you use Ghostview or something of the kind, because you keep getting a little dialogue box popping up to tell you what page it's printing. By default 3col just prints out all the characters you give it. N starts a new column if there are fewer than lines left in the present one.
H produces a horizontal rule of width .
P begins an embedded PostScript object. That's about it, I think.