Steel Channel flex ?

I'm looking at a gantry with a pair of 3" (1/4" web )steel channels
running about 36 inches.

These are starting to get pretty heavy, and if the flex were not too
great, it would be nice to be able to use only one.

Anyone know how to determine what the flex would be on a channel if
the cutter were mounted 6 inches off the face ?

Any suggestions for a stronger frame ? I or box ?

Dave