Asking once, well, in writing

Accept the role first, warmly and clearly, then ask. Something close to: I am glad to accept and looking forward to starting; before I sign, is there any flexibility on X, and here is why I am asking. One request, or a short list, each with a reason attached.

Put it in writing so it gets considered rather than reacted to, and so the answer is on the record. Then stop. Asking once is normal and expected almost everywhere. Asking three times reads as something else — and the thing you are negotiating for is a job you then have to do alongside these people.