Just in case they are ever created as units for an emergent Targaryen invasion... or even as sellswords.
What do you think the three spears, with which the Unsullied are known to have trained, actually are? I can't recall ol' Gurmy ever actually specifying.
The way the Unsullied's fighting style is described leads me to believe that their principle function is to act as a kind of hoplite phalanx, massed in close ranks, forming a wall of spears and shields that is all but impenetrable to both cavalry and infantry and reasonably well protected by their shields from most types of skirmishers. They would have the discipline and stamina to outlast their opponents, blunting any enemy attack and doing most of their killing in looser formation once the enemy routs. The kind of formation you would only really want to deal with by catapult, dragon, or charge to the rear.
So that's one spear. A long shafted spear, light enough to be thrust with one strong arm, long enough to be used over at least two ranks.
But what are the other two? The candidates that come to mind are pikes, javelins, short stabbing spears, or perhaps a different kind of polearm.
Pikes would suit their drill and training well enough, if they could rely on sufficient skirmisher screens to risk foregoing (or just reducing the size of) their shields. This would require a change in their basic equipment, however, which is perhaps more worth mentioning by GRRM. Is there any evidence in the lore that they ever do this?
Javelins would also fit their tactics well. They could throw them before the front lines engage just as the Romans did, dealing a serious blow to enemy cohesion, numbers and morale before the work of stabbing begins. I don't believe they are ever mentioned, but would fit perfectly.
Short stabbing spears. These would be more useful in single combat or if the formation loosens up for pursuit or in rough/urban terrain. For example, once the initial enemy assault is blunted, the Unsullied could switch to these to turn rout into massacre, stabbing and slashing with the tip in more individual encounters, while still protected by their usual shield. Fits well, but again not mentioned as far as I know. I could certainly see this as a prefered weapon of the summer islanders, which the Unsullied would therefore now about. THen again I would expect them to carry daggers or short swords as secondary weapons anyway, so the short spear is not necessarily needed.
Finally, another type of polearm. Most of the opponents you would expect the Unsullied to face do not wear armour so heavy they would need one, but perhaps GRRM's account is as cursory when it comes to the heavy armour of Essos as Europe's collective imagination when considering the heavier armours of the Middle East. It is conceivable that in order to fight better armoured enemies (the Legions of New Ghis perhaps, sellswords who were once knights in Westeros, or simply the personal guard of a Khal who values cataphracts), the Unsullied would drill in the use of some type of halberd. Or they may have had trainers from the Jade Sea, using something like a naginata. The heavier, more versatile head of such a weapon would be a reason not to use it with a shield, of course. But as a tactical option, it makes sense.
I personally lean towards the long spear, the javelin, and the polearm for special occasions. What do you think? What are the three spears?