So yes an FF1+2+3 cart is possible (with MMC5), but you're stuck with the original ROM sizes.
But really... what's the point? Multicarts only really made sense when they were an actual cartridge -- when emulating, who cares if they're separate ROMs?
I agree. The idea of an updated FF I + II was only appealing to me because the original FF I + II cartridge was a standard, retail release. Adding FF III to that cartridge is a whole different beast - not impossible, I'm sure, but a big, big step in another direction. From my understanding, FF III pretty much maxed-out what the NES could do, in terms of ROM size and cartridge space limitations. I feel like trying to force all three games together on a single cartridge would cause more harm than good... but then again, It's probably possible, in some form or another!
Now as far as just patching the standard FF I + II ROM with this current Chaos Rush translation, I'm all for that. Just having both games translated onto a single cartridge is a minor miracle, as far as I'm concerned. The fact that one of the current versions on this site included the B-button dash from the GBA / PSP re-releases, as well as the ability for both games to save independently of each other, to me, is quite impressive. How much more work could really be done to that FF I + II cartridge before it starts to become something else entirely?
That's what drew me to this Chaos Rush project, initially - it's more or less a straight translation of FF II, with no bells and whistles. While I love being able to dash, any major gameplay changes / rebalancing kind of loses me. Getting through the game faster because I can run faster on the overworld is one thing - getting through the game faster because it's been re-coded to be easier... eh, that I'm not really a fan of.
FF II is a tough, tough game, and this retranslation preserves that. While I have played the game on the GBA and PSP releases (and enjoyed them!) I still prefer the Famicom original, just for the challenge and classic 8-bit soundtrack. FF I is a challenging game, too, which, along with their smaller ROM sizes, is what makes them such a good pairing to begin with. I mean, there's a reason why Square re-released the two games on one cartridge several different times over the years, right?
While FF III is a great game, it's also a more complicated one, both in terms of gameplay and graphics. FF I + II seem like games from a similar era, while FF III definitely has more of a "next generation" vibe. Along with Dragon Warrior IV, it's a standout role-playing game from the end of the NES lifecycle. I have much respect for Chaos Rush for even attempting a retranslation of it, and I am definitely interested to see where it goes.