Bathroom Vent Fan Upgrade
You can install a reducer, but you'll cut your flow volume by an equal proportion:

28.3in2 - 7.1in2 = 21.2in2 (an area reduction of 75% when going from 6" to 3" duct)

This will negate a significant amount of your fan upgrade, will make it work harder, and may shorten the motor's life due to reduced cooling. 

You might ask yourself whether you really need 150 cfm (and the accompanying noise).