This marks either the return of Nissan to reliability, of the demarcation of Honda from reliability. Mitsu being a wildcard here, is already a tossup on reliability. I don't think they could manage to sway Subaru into joining, they are already working pretty closely with Toyota, which this looks to be an attempt to compete with.
It could do some things that I feel are less likely though.
Like, help Honda step back into the arena of performance cars (outside the Acura NSX (no longer made), they lack).
Maybe attempt to work together on the Acura and Infinity lines, both of which fair poorly in their markets. I do not see how Mitsu can be useful in that endeavor.
If they gobble up the Mitsu and Nissan truck / heavy equipment knowledge (not the garbage pushed on Americans as trucks by Mitsu back in the day) that could get Honda a real truck perhaps rather than that pretend truck, the Ridgeline. Honda has little truck experience overall, outside of little trucks (Acty), they could stand to gain in this area. All depending on this holding company getting the Mitsu and Nissan truck groups as well as the car groups.
Nissan and Mitsu have lots to gain in software and reliance from Honda, Honda has little to gain from the other 2 except learning cost cutting measures that have brought the other 2 to the points where they are today. Nissan used to be pretty good, but has become garbage in the last 2 decades. Mitsu has always been hit or miss, and worse during the Chrysler years.
There could be some easy parts advantages for all three possibly. Honda makes excellent 4cyl engines. Nissan has great 6cyl engines. Mitsu, makes the better AWD system from the 3.
I do not yet have a clear enough picture to speculate further, its all if and maybe so far.