"Puertos Cerrados" in Buenos Aires
In addition to Casa Saltshaker, we absolutely loved Casa Felix and Almacen Secreto. Diego Felix is a wonderful chef and his garden is lovely. His food is 'pescatarian' with an emphasis on local, and it's a fixed menu. You have an opportunity to meet the other diners over a drink in the garden before moving to your tables. Almacen Secreto has a menu where you can choose from North, South and Central Argentine foods. It was also in the garden and I don't think there were any other tourists there. I really recommend both places. You can google them for the websites. Casa Felix is in English and Spanish. Almacen Secreto is all Spanish, but you can email them in English.
With only 3 nights - wow, it would be difficult to narrow it down.
My family and I have been in Buenos Aires in July in 2008, 2009, and 2010. When we rented an apartment for the first time, the owners made reference to Casa Saltshaker, wich was conveniently near the apartment. It is, indeed, in a home. We have returned every visit. Look for it on www.saltshaker.net and follow the link to dining. We have met interesting people there and have enjoyed it every time. The food is good and you know ahead of time what is on the menu. I don't think you'd be disappointed.