Rebex Terminal Emulation for .NET offers a version for Xamarin.iOS and Xamarin.Android. It currrently only includes the UI-less classes though. If you need the UI as well, it is possible to write your own custom renderer on Xamarin.iOS and Xamarin.Android. See more info about Terminal Emulation on mobile platforms.
Experimental beta version of Terminal Emulation component for Xamarin.Mac has been released as part of release 2017 R3.
The Rebex Terminal Emulation component does not support Xamarin.Mac platform yet. Let us know to firstname.lastname@example.org or leave us a comment here, if you are interested to have binaries for this platform.