It's also possible that all 3 of them win, along with Peter Scholze. In recent years, ICM almost always gave out 4 medals, the maximum allowable. If Simon Brendle gets a medal, then maybe Manolescu will be left out.

In the choice between Manolescu, Williamson and Zhang it will come down to the committees priorities among the following secondary criteria : subject and geographic distribution of medalists, single vs multiple authorship of main work, elegibility in future cycles. It will be a difficult decision. My hunch is that Zhang will get the medal in this group since he may have an edge in the primary criterion, namely originality.