Just got a response on Friday:

"Alexander,
Thank you for writing us! And I am sorry for the confusion. Our standard order of tacos is three, and our menu boards and menus display the price for an entire order. Because we want our customers to eat what they want, we’ll of course depart from our standard menu in any reasonable way, like with single tacos or quesadillas or nachos or whatever you want really with the ingredients that we serve. But if we tried to display a price for every possible variation off the menu, unfortunately, we’d need a “price encyclopedia” in each restaurant, and we wouldn’t be able to find enough space on our walls to display all of the possible combinations of our foods.

If you think $3 for your taco wasn’t a fair value, then by all means, please let me send you a coupon that’s good for a full order of tacos for free. What’s your mailing address? With regard to future menu board innovations, our restaurants are ultimately transitioning to a new menu board that actually has the single taco listed. We may also incorporate other non-menu items too, but that remains to be seen."