If you have a visa for Brazil, you do not need a visa to enter Paraguay or Argentina, when you just go there on day trips. As a matter of fact, there is practically no border or immigration control.
The same should be valid if you have a visa for Argentina.

However, this is just based on my experience (and I do not need visa for either country), and what I have read and heard. I am not issuing any guarantees. I suggest you double check with consulates and/or people who has crossed the border recently, and has US/AUS/CAN passports.