One offer expires Friday, the other company hasn't even finished my final round. How do you buy time?

Recruiter A wants an answer in three days. Recruiter B says they're "moving fast", which so far means next week sometime. I don't want to lose something real chasing a maybe, and I equally don't want to sign and spend a year wondering. Has anyone asked a company to accelerate, or asked for an extension, without coming off flaky? Mostly looking for what the wording actually was.