Jagger, could you give some info on what context the triarii were using a phalanx formation in 280 BC? I'm assuming this was at Heraclea vs. Pyrrhus? The info before the 1st Punic War is pretty sparse. The Samnite wars apparently led to abandonment of the phalanx formation for more flexible maniples copied from the Samnites (I read one bit on the web about the Samnites deploying their maniples in "duplex acies" with a line of skirmishers in front.) The terrain of the Samnite wars was not generally suited to phalanx warfare.

The triarii maniple shown in Goldsworthy's "The Complete Roman Army" is 60 men, but only 3 ranks deep. If the illustration is correct, then it was clearly not a phalanx, and unless the maniples were combined, they would not be able to act as real phalanx. The legions formed on the triarii maniples so it seems unlikely that the triarii maniples were regularly combined to form phalanx formations.

Another argument against triarii forming as phalanx is their normal placement at the rear. Phalanx were typically used as the main line.