I realize the OP only has three rolls and is Japan where he probably cannot easily get Diafine so these comments are just in general...
Plus-X is no longer in production, and had some dedicated followers who still mourn its demise and would probably pay extra to get their hands on these three rolls. Pushing it to EI400 sounds like a profoundly broken plan to me, unless you are absolutely unable to afford or get one of the available ISO400 alternatives (Tri-X, HP-5+, TMY, Delta 400, ...).
I am working my way through a 400' roll from the 90s. It's my experimentation / goof-around film. Using D-76 stock I routinely push it to ISO500. At 500 it is pretty high contrast and the grain is no worse than HP5+. It's a little thin, so enlarging takes a bit more effort, but scans are just fine.
Am I the only one here who has actually used it in Diafine? Throw out what you know about "pushing" - the results won't look right at box speed anyway. It's a really great combo.
I recall someone on the LFPF shooting it in medium format in Diafine that agreed with me when I mentioned it.
I changed to FP4+ which I like, but it doesn't have that versatility to carry one film for 100 and 400 with choice of developer.
Back in the 70's there was a developer called Acu-1, and at that time, rated Plus-X at 320 (in 35mm, which was the rage at that time). That film and developer combo was nothing less than magic. It kept the film latitude, and the grain was like Microdol. The contrast was perfect. I'd use them today if they were still being made like they were then.
Made by the same people who made Diafine, wasn't it? Probably a similar formula PQ developer but in a single solution for one shot. I think Acufine is similar, and still available.
Plus-X was/is also known for keeping extremely well in cold storage. I've heard of rolls from the 70s and even 60s working well with minimal fog but that is, admittedly, just hear say. I do have that Arista stuff in my fridge but I haven't tried any of it since it expired as I all but quit shooting black and white in 35mm once I got medium format cameras, the exception being for very low light and that was TMZ and now D3200.
But I'll get around to trying it, I'm sure.