The title is slightly misleading, the Ender-3 (touted as the new best cheap printed) is not the first open source 3D printer.
Nearly every printer is based on a type of RepRap printer which kind of started the original boom. You can find a ton of open source printers on their site:
https://reprap.org/wiki/RepRap_Options
Nearly every printer is based on a type of RepRap printer which kind of started the original boom. You can find a ton of open source printers on their site: https://reprap.org/wiki/RepRap_Options