How to deal with overly helpful co-advisor?

Set a meeting with all three advisors, explain you'd like to focus entirely on the writing, and ask whether regular meetings can be replaced with less-regular meetings when you need support.

If she isn't a formal advisor you might just get by ignoring her. But if that doesn't seem right, I'd suggest working through one or both of your other advisors. They can quietly speak to her about the fact that her communications are disruptive and not helpful. It might be harmful for you to say the things they they can say comfortably. Let this be a faculty fight if a fight needs to occur, not a student v faculty issue.

This sounds like a rather delicate matter. On the one hand, your request to not have your supervisory team shaken up while you are finishing your thesis is completely reasonable. On the other hand, if she was your supervisor before leaving it can be painful to come back from a long medical leave and see that your projects moved on to the extent that you aren't actually needed (or even useful) anymore.

If you have a healthy relationship to your main supervisor I would have a frank discussion with them. You should not complain, but saying that you feel that this de-facto change in your supervision isn't actually helping you is completely fair. However, as always in these matters, stick to the facts and leave out any speculation about her motives. Don't say or imply that she is desperate to help you or to feel useful. Just say that these additional meetings are draining your time and energy, and that you feel like you are not getting much feedback that is useful to you in this phase of the project.

Maybe your advisor will see the issue and talk gently to Jill, or maybe they won't. In the latter case, simply humouring her and sticking it out for the last few months is probably a better option than escalating this into a big conflict.