How close can I be to a 3" PVC exhaust duct from the furnace with an IC rated can light?

IC means the recess housing can be in contact with the insulation, because the insulation will entrap the heat of the lamp without causing any combustion problems.

I see no problem with the can being at least 3 inches away from the PVC duct, but best to check the installation instructions of the fixture, and the exhaust duct.