Windows 3.11 will run on a 286. It works about as well as standard mode Windows 3.11 on a similar clock speed 386 with equivalent RAM. An early AT upgraded to 1.5 MB of RAM will provide a less than satisifying experience. However, with enough extended memory, and a plan to run nothing but Windows applications, the whole process will be fairly smooth. I did a lot of work with Windows on a 286 with 5.5 MB of RAM.
Note that this means standard mode. No virtual memory, no vxds or other high performance drivers for hardware, any EMS memory needed for a DOS application will have to be seperately defined, and no virtual machines or background processing for DOS programs. I think some of the higher video resolution modes did not work in standard mode.
Windows for Workgroups (also 3.11) was a different product and only ran on a 386.