I have a joint PhD between my university and an overseas university, so in total I have around 4 PI's. I usually communicate and give regular updates to the one who is in my institute but to the rest its usually once every month. The issue is regarding the advisors in the overseas university. One of them replies to my emails but only after sending numerous reminders and the other one doesn't even bother to reply. He was there in my confirmation seminar but he never really communicates. I am supposed to be joining there in Feb 2023 but I am not sure how everything will turn out to be. I am a bit scared as they haven't been that responsive or interested over the emails, but since I am already enrolled in the PhD Program, with no option to change my advisors, what should my plan of action be? Should I confront the one who barely replies? Should I talk to the one who does reply, about the involvement of the 4th PI?
What happens over email is not the same as what happens in person. So your experience with those advisors over email will likely be very different from your experience with those advisors in person.
If multiple people are responsible for the same PhD student without clear agreements about who is responsible for what, then you can quickly end up with a dilution of responsibility. If some are not physically there and there are no such agreements, then you will very likely just fall through the cracks, and you will be just the local PI's responsibility.
That is not nice. It is not professional. It should not have happened. But is not necessarily a disaster either. If this is what is going on, then it is likely that when you show up at the oversees university, you will be seen by the oversees PI as her/his responsibility now (at least for the duration of your stay) and everything will be just fine.
I would just ask your local PI what they think. They know the other PIs better than we do.
I'll give a slightly different answer to supplement what Maarten Buis says. While it is certainly not polite to ignore email from a student advisee, remember that some (a handful) faculty take the "squeaky wheel gets the grease" principle to an extreme. If they feel it's not urgent, or referees / colleagues / students aren't clamoring for it to get done, it just does not get attended to. Your non-responsive advisor might be of that ilk.
From his perspective, you're a co-advised student who is yet to join. He might not have considered your emails to be sufficiently research focused to interest him. He might have assumed that if there are things to discuss, you're likely to do so anyway after you get there. You also have other advisors, so someone else is there to attend to you. Ergo, no reply needed.
Not saying this is the way things should be, but faculty have limited bandwidth and optimize in ways that might surprise students.
Here's a suggestion: Ask for a Teams or Zoom meeting in the subject line of the email, with a research-related proposal or question (which you undoubtedly have) in the body of the email. That might get the guy to take more interest.