It seems to me if you have a spear with multiple blades on the front meant for piercing, you could stab your foe multiple times with one thrust, increasing the chance of hitting a vital organ. There's probably a reason why these kinds of weapons rarely if ever existed and were never used on large scale.
They were used:
Chinese: https://i.imgur.com/bJcnOSN.png
Chinese: https://i.pinimg.com/originals/5c/5a/95/5c5a953dc343cd25b350ede9badfa3a9.jpg
European: http://www.worldsofimagination.co.uk/Forks.jpg (from Stone's Glossary)
European: https://www.metmuseum.org/art/collection/search/26083
Compared to a conventional spear, with a blade of the same width, they might do more damage (if more than one blade hits and penetrates), but
penetration will be worse,
the head will be heavier, and thus the weapon will either be slower, or will be made shorter to compensate,
the wider head is easier to block, and
it's easier for the weapon to be trapped by an opponent.
Penetration will be worse because if multiple points hit at the same time, the weapon will be less likely to pierce armour or thick clothing, and if one point hits first and pierces armour or thick clothing, the weapon will be slowed and the next point that hits is less likely to get through, which will stop the first point that did go through from going any deeper. If one seeks more damage, a better solution is to simply make the spearhead wider. This also adds weight, and reduces penetration (but not as badly), and avoids the last two disadvantages. Generally, a conventional spear (even if narrow-bladed) will do sufficient damage, and one still has the option to simply pull the spear back and hit the opponent again (and again) if needed. Thus, forks/tridents are outnumbered by spears on the battlefield.
The last disadvantages work both ways: it's easier to block and trap weapons with a fork/trident than with a spear. This is useful in a combined arms formation, to stop and trap an opponent's weapon (or the opponent) and hold them there for long enough for your accompanying spearmen to finish them off. This also makes them useful off the battlefield, for bodyguards and policing (and in these contexts, the opponents are less likely to be armoured, and reduced penetration is less of a problem).
Note that the Chinese examples and many of the European examples have a longer central spike which won't suffer reduced penetration - it will go deep enough to be effective before the side spikes hit. On the Chinese ones, the side spikes are often curved inwards to help trapping. This makes the whole head quite wide and heavy. A lighter head can be made with outward-flared side-spikes: https://www.metmuseum.org/art/collection/search/26088 - side-spikes like this are not very effective for thrusting since they aren't in-line with the haft (even if the central spike wasn't so much longer), but provide more width for catching an opponent's weapon.
