To review:
You have a bathroom vanity light fixture with 3 light bulb sockets,
and the neutral wire is at the fixture, not at the switch.
You were using an X10 PLW01 wall switch with incandescent bulbs,
controlled by an X10 HR12A remote.
Then you changed to MaxLite G25 LED dimmable 10W (100W equivalent) bulbs,
and now the X10 PLW01 wall switch doesn’t work with the LED bulbs.
So now you are having an electrician run a new wire to get neutral at the switch,
and want to know if the X10 XPDI3 dimmer switch will work.
Specs for the XPDI3 say minimum 40W, and since you only have 30W, THIS IS LIKELY A PROBLEM. The XPDI3 is not a true rocker, it is a pushbutton dimmer switch with a paddle that overlays a single pushbutton hidden beneath the paddle. The XPDI3 is a receiver only … it responds to X10 commands and controls the connected load, but it does not transmit X10 signals when the paddle is pressed.
A better choice is the Insteon 2477D Dimmer Switch. The current production version, Revision 8.2, still supports X10, and it even supports X10 Scenes. It is a true rocker dimmer switch. It has LEDs to indicate the level of brightness of the load. It is both an X10 receiver and an X10 transmitter. It can be set to have any default level of load brightness, and any ramp rate.
Furthermore, since you are using an HR12A remote, with the 2477D’s X10 Scene capability, you could have different buttons on the remote correspond to different levels of brightness. For example, using an “A” housecode remote, you could have button 1 (A1) correspond to full brightness, button 2 (A2) correspond to 70% brightness, and button 3 (A3) correspond to 20% brightness. You will need an X10 Maxi-Controller (or equivalent controller) to set up the X10 addresses in the 2477D.
I have a 2477D Rev 7.8, wired to a fixture with 3 Cree 4Flow LED dimmable 6W (40W equivalent) bulbs. I’m using the 2477D in X10 mode only (not Insteon mode), and I also have an X10 Scene address set in it. It works perfectly.