I gave 3 months notice at my job to my manager, but he's still assigning me more long term work. What should I do?

I told my manager that I'll be leaving in 3 months a couple weeks ago, so that he could be prepared and not include me in 2022 planning. He was sad, but understood. He tried to change my mind this week, but I told him that I'm not planning on staying.

But, now he's making me the manager of a few more people, and talking about projects for June of next year in our group meetings. He doesn't want me to share with the team yet (to avoid awkwardness and because our team dynamic is kind of delicate), but how do I onboard these new team members and try to do right by them, when I know I won't be here for much longer?

Such a long notice was very considerate of you! It is really odd that your manager is doing this but ultimately you have done your part and notified the company and your manager, so I'm not sure it's on you to solve the problem of what happens when you leave.However, I understand it might be unfair to people you are managing, I would think the best course of action is document as much as possible about each of them and your work so it makes transition easier and for someone to onboard easily.
Until you actually leave, this will continue, and to be honest, I don’t blame your manager for acting this way. You’re still on the payroll, and until you leave, he wants his money’s worth. Since you haven’t said publicly that you’re leaving, of course he’ll include you in planning. Why wouldn’t he? There’s no way to solve it, now, except to leave. The team dynamic is not your responsibility. They’ll adjust.
Unfortunately they will keep giving you the work and I'd suggest you continue to deliver even if you don't feel like it, you don't want to give them the opportunity to poke any holes! A recommendation, in the future I'd advise you to give a 2 to 4-week notice next time!
Bring HR into the conversation, tell them everything you said above and then after that meeting memorialize it in email. CC your personal email in BCC and openly CC your work email. Any time you go into a meeting with your manager ask for HR to be involved if it does not change. Tell him that your leaving is not negotiable and that you do not feel comfortable being in interviews unless you can tell folks you are leaving. The emotional state of your delicate team is not your problem. Step back from it and don't let him gaslight you. Good luck in your new role!
Honestly, I agree with everything everyone has said. I’ve been in a similar situation and unfortunately that’s just how it’ll be until your last week when your departure actually dawns on them. Create docs to aid transition and help the people you’re currently managing but remember that in as much as you feel bad, their including you in long term work is not your responsibility. You’ve done your part (which was so amazing of you to give a 3-month notice) and the consequences of them still including you in their long term planning will be theirs to face. Makes no sense but that’s just how it goes in these situations.
Unless you have an Executive role, it takes a lot of caring to give a 3 month notice period. The problem is, in doing so, you've taken on more of a burden than you needed to. I have some experience and lessons learned with this, I gave 7 week notice to enable a higher degree of knowledge transfer, given I was working on a lot of projects that no-one else knew much about. Guess what? Management did not support using the extra time for this purpose, instead they just kept assigning me new work right up until my last days. In hindsight, I think they were doing so in the hope that I would find something they were giving me interesting enough that I would stay. Your boss may be doing something similar.Not telling your colleagues is a problem. It won't be real until you are able to tell everyone, for you or your boss. I strongly recommend to work out a firm timeframe for telling everyone. Your boss may think he's protecting people by not burdening them with this info, but there is going to be more awkwardness, not less, if they find out that you have both been keeping this info from them for an extended period of time. I suspect that he's trying to make his life easier, more so than theirs, by holding onto this info. The more time you have to plan together, the better the transition will be. More importantly, the sooner everyone knows, the sooner that your transition out will actually start in a meaningful way.Ultimately, for as long as you are an employee there, you have to work with your assigned tasks, but you might want to talk to your boss about the following:1) Succession plan - who is going to replace you? New hire or promote from within? Depending on your current level of responsibility and the relationship, you might be able to persuade your boss to enact the plan sooner vs later.2) Knowledge transfer requirements. If there is not established processes for capturing and transferring role and assignment knowledge, what would your boss like to see for that (e.g. types of info captured, docs vs videos)? How much time would he like you to spend on this each week vs normal responsibilities?3) Risks. Again, depending on the relationship with your boss, you might want to have an open and direct conversation about risks associated with your transition, as well as some suggestions about how these might be mitigated for his consideration, feedback, and action.Keeping your boss focused on the fact that you are leaving, and this is a reality that he has to deal with will be important, especially while you're in the twilight period where you're not permitted to share this with your colleagues.Otherwise, do the work and manage the team that you're given, document what you can, and let the rest go. It's your manager's responsibility what happens after you leave, not yours. Focus on self-care. By opting for a longer transition time, you have chosen a more difficult path. Acknowledge that your self-care needs will be higher and you have a responsibility to take care of yourself as you go through this process. Good luck!