How do I tell my adviser that group meetings may be too time-consuming in my last year of my PhD?
You don't tell him that at all.  You go and contribute as best you can.

Even if the meetings are of no value to you, your presence may be very helpful to your adviser and the newer students. That alone should be sufficient reason to attend. It's not always about what you personally can get out of something. Sometimes it's about what you can contribute.  It's also sometimes about just doing things for people who've done things for you.

If you feel the meetings could be better run, consider what you might do to become a more helpful participant, facilitating discussions and consensus outcomes that don't take 3 hours to get nowhere.  Ask questions that invite others into the discussion, offer your own ideas, propose plans or assignments.  If you were there 3 hours and nothing happened, consider the possibility that may also have been 3 hours when you didn't do much either.