With most amateur radio mobile transceivers, the fan comes on using either of two methods:
1. when a temperature sensor indicates that the radio's temperature is above a setpoint, i.e. when it gets hot,
--or--
2. the fan comes on when the radio is keyed to transmit and the fan stays on for a set period of time.
The FT-3207 is a fairly new radio, so there may not be many people who have one and who can give you a definitive answer.
You might consider contacting Yaesu Customer Service and Tech Support directly at 714-827-7600 between 8AM – 5PM PST.