With any borderless printing you have to select a paper size that is explicitly named "Borderless".
In the Print Module, left side bottom, just above the film strip click on <Page Setup>, and in the dialog box under Paper / Size select a paper "Borderless A3".
The paper named "borderless" includes instructions for the printer to print to the edge of the paper. Without it your printer will always print with boders, and in Lr you cannot resize the image cell size to the edge of the paper either.
Naturally, this works only if your printer is capable of borderless printing.
If you find the paper "Borderless A3" listed in your paper sizes in the printer dialog - accessed via <Page Setup> in the Print Module - then your printer should be able to print borderless. I cannot imagine that your printer comes with "borderless" paper sizes installed and is not capable of printing without borders.
O, did you install this special paper size yourself?
Check the specs of your printer on the Epson website; it should tell you if borderless is possible with your printer.