Will the AI join an MA by offering them cities, or doesn't that work?
AFAIK, in C3Conquests you can only gift cities, or trade/demand them during PT negotiations. If you try to 'sell' cities in exchange for anything else (like an MA), "they will never accept this deal".
What you can also do, rather than signing an MA, is to gift away a newly captured city to a neutral (but weaker) third-party, carry on with your war, and then go back and (re)capture it later, e.g. if it flips back to your enemy, or if they DoW the neutral party to try and recover it (having your enemy DoW the third party is obviously the preferred outcome here); or once your enemy's dead, you can DoW the third party yourself. Gifting the new town
also frees up combat-units that might otherwise have languished on garrison-/ resistance-quelling-duty, and can slow down enemy SoDs, as they move in and out of the new foreign borders within their former territory. (You can also use city-gifting to instantly teleport [injured] units -- including Slaves -- back to your capital from anywhere on the planet, although that is kind of exploitative.)
You also cannot buy an MA against any Civ with whom you currently have a PT: you have to be at war already (you
can DoW and buy in allies on the same turn, though). But if you
are at war, it's usually pretty easy to get another AI into an MA -- especially if your ally has fought your enemy before, and/or is bigger/better-armed than them. Rather than upfront goods (techs, lump-sum gold), I prefer to pay per-turn for MAs, and preferably only gold, even if it works out at slightly more expensive per-turn (e.g. I'd rather pay 10 GPT than 180 gold up-front). That way, if (or rather, when)
they break the alliance prior to the 20T-expiry, they get no lasting benefit from it, and I only pay for the 'help' that I've received. I might also offer Iron
or Horses
or Salt as possible sweeteners, which will help the war last longer (and I'll sell my Luxes separately for GPT -- hopefully getting back the GPT I'm paying for the MA -- so my income won't be affected by them making peace).