Here's my thinking... To handle 3m length on X with a 500mm wide gantry (to allow for a 220mm wide cutting area) you're going to need to use a fairly heavy ballscrew or go for a belt drive. Either the gantry sides are going to be tall to accommodate the 220mm workpiece+the tool length or your going to have to raise the workpiece manually. The actual required Z motion is small, the max wall thickness of the tube.

By making the X run alongside the workpiece the saddle will be much lighter and relatively narrow. The workpiece supports can then be made to manually move horizontally to handle different diameters, much easier than doing it vertically. The Y axis is then only the cutting depth of the spindle in and out of the work and Z only needs to raise/lower the horizontally mounted spindle 220mm or so. All in all much easier to make and much more rigid for a lower weight.