My navy is good enough to eff over Ming and Qing's at once, Fang not included as I'm not sure whether they even have one. On the other hand, the ledger tells me that my enemies have about 200k of troops between them and around 150k manpower, so we're looking at three enemy soldier for each Japanese - I am higher quality and have better generals, which should shift the disadvantage to about 2:1. Add to that lack of AI coordination and I should be fine. If I am not, I will sacrifice my vassals, Xia and Khmer. (Leftovers of the Korean empire that give me more money this way when compared to horrible AE penalites. I went straight from royal marriage to inheriting for some reason.)
This is not the first time I've been in this situation, although this time, they are not in another war in the first place - however, their westernmost provinces should be very revolt prone, I'm hoping for indirect support from there. It will be fine (hopefully) and I will go away with some more chunks of Manchuria. That's how I roll here. Just let them come to Korea and let attrition help. I play on hard here, increased after I found normal to be too easy. High AI aggressivity, too...
Ahem.
I can wipe the first Qing wave before their allies even arrive. They are invading with 20~30k at present and superior Japanese discipline/shock six generals should work wonders here, even though we are not mobilized at present. (That should be even better once my King dies, the Heir has military skill 8 and I can enact both discipline decisions at once therefore, giving me another 10%ish discipline (minus one move towards quantity) and one more leader shock, not to mention extra morale from the eight stars themselves as compared to the four my current ruler has as his weak point. +4 Land/Quality right now, though) At first Ming will be distracted by my allies and Fang will be distracted with staring over the narrow strait as well. I could also take Ming's islands to increase warscore, blockade their south and eventually WP them out, OR leave them for last and then balkanize.
If I beat up one of them enough some horde might declare, opening a second frontline. However, at that point, I may very well win myself.
The goal of this war is to take Qing's eastern coastline as well as the two Manchurian provinces I need for my current mission, WP out Ming or make them release one vassal or two in the ideal case, and then force-vassalize Fang. Once Ming is out Fang will be a cakewalk, they have about 25k of army. Fang will not be able to make effective use of their army, hopefully.
...should cover it. I am excited how this plays out!