I have a harbor breeze cieling fan with a 3 speed pull switch. It has 4 wires black /gray /brown and purple. The switch has L/1/2/3. I need to know which wire goes in which position ?

Old switch and new switch have to be compared using multi-meter or continuity tester.
Each 3-speed switch has identical pull-chain sequence: off-high-medium-low.
Each 3-speed switch has only one wire that connects to Hot wire.
But the other three wires can vary.
For example connect power to switch Hot wire. Let's say switch is in off position. Pull chain once for high speed and power might flow to just one wire, or it might flow to two wires, depending on the switch.
Pull chain again for medium speed, and power might flow to just one wire, or it might flow to two different wires.
Pull chain again for low speed, and power usually flows to just one wire.

Easiest place to start is to find which wire is Hot.
Only one wire connects to Hot power.
That same wire on the new 3-speed switch will connect to all other wires. When testing, only one wire connects to all other wires.
Once hot wire is determined.
Then test which sequence of wires receives power each time chain is pulled.
Repeat same test on old switch.
Note how old switch was connected. And then map the change-over to new switch.

Open following link for resources:
http://waterheatertimer.org/How-to-wire-Cooper-277-pilot-light-switch.html#3-speed