Sure. You can use any widget, for example one of the knobs. Select “MIDI In” as plugin and Channel 1 Output as parameter. Then you can change the destination MIDI channel with this knob and change and recall the widget value with Rackspace variations.
That being said I don’t recommend this approach as you can easily get hanging notes. In this case I would just use three different MIDI In Blocks and change the “Block MIDI In” setting with three different widgets and three variations.
“In reality” I wouldn’t do it like that if I had 16 instruments. I would either use a keyboard that lets me change the transmit MIDI channel (and takes care of pending note offs to avoid stuck notes) or create 16 rackspaces with only one instrument loaded in SampleTank.