As I read the web site there are two baggage plans: 1. By weight 2. By piece.
The Brazil-Argentina flight is considered a "regional" flight and falls under the weight category. As such you are entitled to 20 kgs each (the baby does not have any allowance except for carryon*). Since you are traveling as a couple you are entitled to the combined weight of 40kg which can be distributed between your three bags. I would call to clarify this though. The carseat should be able to be carried on if it will fit in the overhead...worst case they will gate check it and you will have it immediately upon arrival.

*"Infant: Not entitled to baggage.
Only an umbrella stroller or collapsible stroller that may be taken on-board the aircraft is allowed, subject to space availability."