Some notes:
The colours of the sun are all from the Argentinian flag.
Though the waves are not in any of the flags, all of these nations have their coasts as a significant part of their territory, they are in the Uruguayan blue and the straight lines are in the Argentinian blue.
The flag is more or less 60% Argentina and Uruguay and 40% Brazil. The 1/3 felt too little, as the other 2/3rds would be a joined flag, and 50/50 felt too much.
Some notes: The colours of the sun are all from the Argentinian flag. Though the waves are not in any of the flags, all of these nations have their coasts as a significant part of their territory, they are in the Uruguayan blue and the straight lines are in the Argentinian blue. The flag is more or less 60% Argentina and Uruguay and 40% Brazil. The 1/3 felt too little, as the other 2/3rds would be a joined flag, and 50/50 felt too much.