I'm gonna switch it up now. 3 way battle.Old and new UNSC and Republic vs old and new Covenant and Sith vs Cortana, her new lame boyfriend, and droids across the Star Wars galaxy. And there isn't gonna be any flood so don't bring that up.
UNSC (I assume you mean composite by new and old right?) vs Republic, stalemate until the Republic reverse engineers Slipspace drives. The UNSC doesn't have the numbers to take out a galatic entity, ships simply beyond anything the Republic has to let them win through superior firepower and defense, or the Intel needed to know what the important targets are. Meanwhile the Republic can't strike back until they get Slipspace drives which is dependent on how long it takes them to salvage one from a wrecked UNSC ship or they capture one through boarding. After that it's the Republic's game to lose.
As for the second scenario, same as the above but it happens faster due to the Sith being more aggressive, better force powers, and larger numbers as they have the Sith Empire, CIS, Galactic Empire, and the New Galactic Empire.
Third scenario, Cortana should
win. Her Guardians shouldn't have any issues shutting down the Droids en mass with their "you don't have technology anymore, have a nice day" pulses they use in addition to her deciding the battlefield thanks to her forces' FTL. Additionally she has access to Forerunner technology. Of course whether or not she has the common sense to upgrade her forces like no tomorrow is unknown. Another thing to note is that her Prometheans are superior to the droids. How you ask? They can all be literally built out of the dirt under their feet, can regenerate, teleport, shoot weapons that either track their targets or ignore their durability, switch their weapons whenever they want (as in a Knight can turn a Scattershot into a Binary Rifle), and generate new weapons. Meanwhile Droids can't do any of that. Of course they do have superior numbers to what Cortana has but their issue is that they lack the logistics to transport them all which is one of the reasons why the CIS lost the war or strike back.