How about "I will schedule it for before three this afternoon," or "I will schedule it for shortly before/after three this afternoon"? My gut says they are all fine. Can I trust my gut?
How about "schedule it for three minutes after/before the operation starts," and "schedule it for when you start the operation"? Oh, what about "schedule it for while you operate that/while you are operating that"?
Also: schedule it for when the doctor arrives; schedule it for soon after the operation; schedule it for sometime; schedule it for a few seconds before midnight; schedule it for the last Tuesday that I am here; schedule it for the near the beginning of the Third Millenium, etc. etc.