It will move in both directions with a single three bearing unit, however any flexing of the unit will cause inaccuracies. Unless there is additional support to prevent flexing (tipping, wag) a six bearing double unit is much more reliable. I would recommend the double unit. Most commercial units have six bearings or more.
I will be posting a much smaller unit using 623 bearings on an 8 mm rod similar to Natko's with a tension adjustment feature soon.