Epson puts limits on what the Eco Tank printers can do. Especially on larger paper sizes. For example, you can't print borderless on normal paper sizes. You used to not be able to do borderless at all, but they have recently relented and allowed some borderless on ET printers, but generally only on photo sizes (4x6) and never (to my knowledge) on anything larger than letter/A3.
Another feature that's locked out for some ET printers is, as you noticed the print quality setting. It might be locked to certain paper types. You can try to select Matte Presentation Paper as the paper type and see if it opens up the capability for high quality at A3. If this works, and you are actually using plain paper, then I'd suggest reducing the print density by about 15% or else you might get too much ink saturation. If your printer has that setting it may by under Maintenance->Extended Settings.
Epson doesn't want Eco Tank printers to be able to do things that actually use large quantities of ink. They want the kudos for offering ET printers, but then take away the features people would actually use them for. Like people or companies that do a lot of posters or borderless proofs, Epson wants them to fork out either for a professional wide-format printer, or to buy like a WF-7820 and then pay through the nose for tiny ink cartridges. TBH, I am really quite fed up with Epson's marketing this way. With one side of their mouth they say "look at our Eco Tank printers, see, we aren't trying to gouge on ink", and then with the other they take away basic printer features on those models. It's also why no Eco Tank uses pigment ink for colours. It's all dye.
The printer itself is actually quite capable of printing in high quality on A3. The printer is also physically capable of borderless all the way up to A3. They just restrict access in the driver. You can hook up the printer to drivers for, say, an WF-7710 in Linux and get those capabilities back. At least, you used to be able to. I haven't tried with that model in a while (I don't actually own one myself) and Epson has been using firmware updates to lock out access to features they don't want you to use.
