There are no special visa rules to this area separate from national laws of the three countries meeting here and forming the Tri-Border, Tri-Country, Three-Nation-area of the Tri-city.. Regulations as to what kind of visa you need will obey the immigration and migration laws of Argentina, Brazil and Paraguay. To be on the safe side you would need to have the three visas if you plan to go around in the area.
This sounds like stupidity to repeat that you will need a valid passport to come here. You will need your passport and you will need to check whether your country needs a visa or not. For example, American, Australians, Canadians, New Zealanders need visas for Brazil. No passport is needed from Argentineans, Paraguayans and Uruguayans (The founding members of the Mercosul / Mercosur) and now the tendency is that Bolivians, Peruvians, Venezuelans may not need a passport. Things are in a beginning stage though. You know, nnnn’s Law tends to be proven true around here. Chances are that whatever will go wrong will go wrong and a Peruvian may find a Brazilian police who has never heard about this no-passport need. I think that traveling is a risky business and travelers are subject to all kinds of accidents like political… So have your passport with you at all time and never, never outstay your stay permit. Say you are given 90 days, don’t overstay those 90 days. It is serial business.
Revenge and nationalism
Brazil demands visas from visitors from all those countries that demand visas from Brazilians. The principle is called ‘reciprocity’ and I have seen a lot of reciprocity problems. Biblically speaking that’s an eye for an eye policy.
I'm working on this!