The short answer is . TN3270 Plus is a commercial product developed by SDI USA. It is proprietary software that requires a paid license for long-term use.
Beyond TN3270, it supports TN5250 (for AS/400), VT100, VT220, and SSH. tn3270 plus free download
Mainframe environments often handle sensitive data. Using a compromised emulator can lead to credential theft. The short answer is