The mayor has said this before.

I pressed Mulgrew this a.m. about the 3% number. He said he was flexible on it, it was the city's number and not his, and that the UFT had actually proposed a geographic system for shut-downs. Mayor came back with the 3% citywide trigger. https://twitter.com/elizashapiro/status/1329453463676346375

As Eliza's thread points out, the pressure the union was putting on City Hall with a strike threat certainly influenced the selection of a conservative threshold. I'd imagine the intense pushback he got from other quarters did too -- re-opening was not super popular.

But it seemed clear to me Mulgrew wasn't married to the 3% figur. Basically he said they showed it to the doctors they were working with, who OKed it, and they went with it. Today he argued regional approaches are better, especially if they include shutdowns beyond schools.