I prefer my cyclocross bike for commuting -- because my shortest/safest path to work involves gravel trails. It's what I use most of the time because it's got the best balance of riding surface versatility and speed.

I will occasionally use my road bike but avoiding the gravel trails extends my commute by 2.5 miles each way (not a big deal).

I use my MTB when the weather is sloppy.

All have 27.2" seatposts, so I just transfer a Topeak MTX beam rack between them and voila! instant commuter bike stable.