Tres Rios is an upscale taco restaurant in the South Side; it is also the first in that neighborhood, I believe. At around $3-4/taco, however it appears to be much more reasonable than other upscale taquerias such as Bakersfield and Condado.
My friend and I wandered the quiet streets of South Side (it was only 6pm on a Friday night) and a waitress from 3R enthusiastically beckoned us to give the restaurant a try! 3R has a pretty hip, modern-industrial vibe and seems that it would be a great place to get drinks!
I ordered the fajitas, which felt to be a bit small if it had been intended to be shared among two people (monetarily it does require two people to foot the hefty price). Nevertheless, the food was hot and the I did like the guacamole, which was refreshing and had bright notes of lime juice. I can see myself returning to try their many tacos.