This sounds like the perfect application for a PIC microcontroller. Set up an interrupt on Port change, flash the indicators 3 times, then check to see if the port pin has still changed (is the direction stalk still hit). Continue flashing if it is, stop flashing if it isn't.
Failing that, set up a dual 555 timer with the first one (as a monostable) triggering the second one (as an astable). Pulse the first one low, the output goes high which drives the second causing it to oscillate. If the pulse to the first is kept low, the output of the first will remain high keeping the second one flashing. If the pulse disappears, it acts as a normal monostable and its output goes low after a predetermined time, switching off the second.
If you understand that, you will understand anything