The IBM 3720 was a communications controller (front-end processor) made by IBM, suitable for use with IBM System/390. The 3720, introduced in 1986, was capable of supporting up to 60 communications lines, and was a smaller version of the 3725. Official service support was withdrawn in 1999 in favour of the IBM 3745.
The IBM 3720 is unrelated to the similarly-numbered IBM 3270 display terminal system.
References
IBM 3720 Wikipedia(Text) CC BY-SA