Alright, let's think about this:
Flood - controlled by singular hive consciousness, rampant expansion, tissue claimed but powered by own bodily processes, evolutionary forms dependent on host, directed by singular purpose, vulnerable to ballistic and conventional weaponry, spread by spore organisms.
Necromorphs - controlled by collective hive consciousness, rampant expansion, tissue code overwritten and largely independent of bodily processes (parasitic development), evolutionary forms independent of host (rendering models aside, no one human body has a predisposition for bile regurgitation or explosive tumors), undirected independent action - instinctive, resistant to standard ballistic shock and electrical impulse - but vulnerable to total nerve shock (fire, pressure) and immobilization, spread virally by contact - hypothetically able to form memetically if host has submitted its body to ritual and the psychic wave rewrites their code to become susceptible to Necro influence.
Tyranids - Controlled by singular intelligence, though formative based on immediate vicinity of broodmates - even psychic pack leaders theoretically perform worse independently. (Lictors exempt), procreation limited to hive ships and spawn pools, meaning raw material must be stored or extracted on site, evolutionary forms written based on encountered threat, though some gene-stock affects designation in purpose (Ork biovores, SM Tyrant Guard, human Genestealers, etc). Instinctive action unless directed by Tyrant or Zoanthrope, meaning vulnerabilities without proper command structure (like Slivers, the more, the horrifyingly merrier). Tyranid spores can affect organisms as mutagens, but transforming a population is not a material purpose of Nid invasion - suppertime is. Greatly resistant to physical and some psychic shock, bred for war.
Geth - Governed by collective consciousness, a democracy of collaborative programs. Expansion protocols largely unknown, but programs able to write and inhabit any suitable vessel and manufacturing platforms seem easy enough to establish - with purpose, Geth can self-replicate indefinitely, so long as materials are available - they only need physical host machinery to inhabit. No real evolutionary measures taken, simply improved host bodies given on feedback data - adaptation not permanent, merely driven to specific finite objectives. Resistant to physical shock, taken out of Mass Effect universe, proven hardy against most weaponry - suspect critical weakness versus EM attacks. Synthetic life form gaining complexity through local presence of its peers. Not as hardy as its listed biological counterparts, but parallel in method.
SO!
In this test of wills, the Tyranids are the superior war force, as the Flood would attempt to control the 'Nid bodies, encounter the psychic presence of the Fleet Minders, and either turn as many grunt units as it could against a foe ever adapting to the Gravemind's own methods, or be gradually devoured as its Flood bodies would be processed on gained ground to make more troops for Nids. In a war of double turnarounds, the better-prepared force emerges victorious. The necromorphs would not fare well against Nids, save for those who are left unattended by their Psychic shepherds, and any tissue found to be converted would be claimed by the gastric pits. All would be consumed. The Geth may come out the best in this, as there is precious little the Tyranids may take from them (same for other 2 counterparts), though if they put up a fight rather than flee, they would be dealing with a form of weaponry that would bypass kinetic barriers and eat away at them. Plus, what happens when a Zoanthrope psy-lances its way into a Geth collective? Do they talk? Or overload and expire?
Now, Nids aside, the Flood is at a disadvantage against the Necros, because for the two to compete, there would have to be a neutral party (soft squishy humans) to exploit. As such, any deaths incurred in the take-over go to Necros, while the Flood needs living hosts. Undead Flood bodies may give the Marker spawn unforseen advantages, and the psychic pressure from both sides may cause breakdown in Flood living tissue (confusion/rebellion even?)
And speaking of the Marker, it is entirely possible that the Gravemind present on this hypothetical world would make the same mistake Humans keep making and hear its murmurs through its 'children.' The parasite would then attempt to bring it closer for examination, listening to its promises, and any creatures it held in any sort of esteem or even its own fellows would tell the Gravemind that Convergence is a really, REALLY neat idea. So, it would butcher its own flock to make more Marker kids, likely take its own... life? and generate critical mass to create a huge-o DEATHSPLOSION! Altman be praised.
