My opinion (I don't claim to an expert, but I try to base my opinion on Ota's (2015) revision of the genus Metynnis).
1st - A female, which could be M. maculatus. An additional lateral picture, showing the fish flat showing the profile of the forehead may help in being more certain. (Current picture does not show the forehead laterally)
2nd - A female M. hypsauchen.
3rd - (again, not a flat lateral picture, not showing well the forehead, nor the anal fin, both of which would be needed to be more certain).
It appears to be a young male, which could be M. maculatus (but I don't think so as it is not spotted), but could also be M. altidorsalis. Again a flat picture would help.
Note that M. argenteus Ahl is not a currently accepted name within Metynnis. That name is now (mainly) under the synonymy of M. altidorsalis, but has also been used with misidentified fish of several species in the genus.