My thoughts on tri-cut tips are that it is relatively easy to grind three flats on a spear tip, once you have mastered the art of re-grinding a tip and not your fingers. Ideally you want the intersecting point in the centre, so the flats have to be three equal-sized triangles. A dual grind tip would be a chisel point, not what we really want (unless as a flat fish backbone breaker)! An even quad grind tip is a bit harder to do by hand, so mistakes whittle down the length of the tip forwards of the flopper hole. Plus too many leading sharp edges decrease the contact pressure on each edge, like the "lying on a bed of nails" trick, but a lot depends on shot incidence angle as not all edges come into contract with the victim instantaneously.
In the fifties and sixties there were all sorts of screw-on removable speartips, "Cutlass" tips like a broad head arrow and the well known original "Mako" with a quad-shaped cast steel head ground to provide four cutting edges with a recessed valley between each of the edges. There were also pyramidal heads with four triangular flats, grinding to re-sharpen them meant that you ended up with a "bent pyramid" as you had to change the angle of the flats or end up with no head left after only a few grinds. Fishing around reefs always exposes the speartips to damage, particularly if you shoot clean through a fish or worse still, miss! These big removable speartips all added to hydrodynamic drag, but fish were generally shot at close range or had the mass of a long 3/8" diameter shaft working as a pile driver behind them. The aim was to kill or cripple the fish immediately rather than secure a hitching point and tire the fish out on the shooting line-tether system (if there even was one). I think that float system tethering has made one of the largest changes to spearfishing as instantly killing (stoning) a very large fish needs a lucky shot or a powerhead on some tough bodied specimens. Powerheads are now not an option and their use has discontinued except for defense against predators (or as a support to one's morale).
Pencil points withstand accidental hits on rocks better and even with the actual point blunted can still be driven through a fish if you are close enough or the gun is powerful enough. From a physics viewpoint the pencil tip concentrates all the initial impact energy at the extreme tip point, but after that the conical body of the tip has to shove meat and scales aside as Don Paul says. The tri-cut has sharp edges that slice like a small knife edge, but the slicing action is limited as the cutting edge is steeply inclined to the surface of the victim. If you lengthen out the taper of the facet grind you can achieve a very sharp tip, but not a very durable one as the cross-section of metal gets very small near or at the tip end. There is more metal in a cone (circular cross-section) than a pyramid (triangle or square cross-section) for the same taper.
Ultimately tri-cuts are a compromise between penetration and durability of the tip and the ease of re-sharpening it, especially as with an integral tip you are working on a shaft and not an expendable tip that can be replaced relatively easily if you are carrying spare tips with you. My fumble fingers have sent more than a few of my spare speartips to Davy Jones' Locker without them being used, especially over a weed strewn bottom, or when I foolishly stuck them in the top of my dive boots and felt water drag the boots open at the top. My wet suit cuffs were already spoken for, car keys in one and the loader for my small spare pneumatic gun in the other.
Adding a grouped speartip photo, from left to right; small tri-cut, Undersee "Mako", Sea Hornet "Mako", later Sea Hornet "Mako", Scubapro tip (both slightly worse for wear) and Lyle Davis conical tip.
