I did the same with my three-speed. It came as a three-speed, and I figured if I was going to climb hills or haul cargo with it, it needed lower gears. I have the standard 48T chainwheel and 18T sprocket. I changed the rear first to a 22T and later to a 24T. Now my 2nd gear is lower than my first gear used to be. I'm tempted to label my shifter 0, 1, and 2.

My top gear is pretty low now, so when going down hill and reaching a certain speed, I can't pedal any more, so I just coast. I think this is a great way to put a three-speed to use.