Revise & Resubmit with Exclusive. When to notify other pubs?

I received a revise & resubmit request from a publisher. They included a list of "issues" & wanted to know if I was agreeable with making changes. I said yes. They are in the process of drafting up a revision letter. Once I address these issues, they want an exclusive on the revised MS.

Question: my MS is currently under consideration with a few other publishers. At what point do I notify the other publishers of the exclusive? Now? When I receive the revision letter/While I'm working on the revisions?

Thanks in advance for any advice.

Revise & Resubmit with Exclusive. When to notify other pubs?