Confused by extra wire on Legrand dimmer
Common = LINE, 1-Pole = LOAD

For your new dimmers in a single pole installation, the Common wire connects to the LINE side wiring (whatever wires the old dimmer's black wire connected to, most likely), while the 1-Pole wire connects to the LOAD side wiring (whatever wire the old dimmer's red wire connected to, most likely).  The 3-Way wire is left unused/insulated, just like the red/white wire on the old dimmer, and the Neutral wire is bundled in with the rest of the neutral wires in the back of the box.  Note that on the Legrand adorne line, the frame has a ground wire that must be connected to the box ground wires in order to ground the dimmer properly; simply connect it to whatever ground wire was grounding the old dimmer.