In Brazil, the currency is the Brazilian real (R$), in Argentina is the Argentine peso ($), and in Paraguay the official currency is the Guarani (₲). However, it is common for tourist establishments in the trinational region to accept all these currencies and also the US dollar. For example, if you go shopping in Paraguay, you can make the payment in Brazilian Real, Argentine Peso or US Dollar, in the amount of the exchange rate practiced by the establishment.
In any case, don’t worry: in the three cities there are many currency exchange houses. We only suggest that you always have a certain amount of local money with you wherever you are, as there are specific cases, such as the Argentine side of the Falls, which only accept payment in Argentine pesos (if cash) or credit card.