Den of Geek reports that Mycroft himself has confirmed that:
Martin Freeman’s rumour about a potential one-off Sherlock film being on its way before series four is bunkum. “There was talk amongst people who have nothing to do with Sherlock I’m afraid”, Gatiss joked.
The plan is still for a three-episode fourth series, and the complicated scheduling is in the process of being worked out, Gatiss says. The team is “moving closer to agreeing dates”. “Benedict and Martin’s schedule, Steven’s schedule, my schedule, everybody’s schedule – it’s like a jigsaw puzzle” he told reporters.
I think I speak for everyone in saying screw everybody’s schedules! More Sherlock and soon should clearly be the priority!