Many thanks to athina1962 for sharing a photo on instagram. The comment with the photo says that they might be shooting in Central Park adjacent to The Plaza. Note, this is not confirmed.
Update: Many thanks to surfingvfx for sharing a photo on instagram along with the comment below. From his bio, he does visual effects.
View from the #gym before shooting #johnwick3 tonight #filmmaking #vfx
Update X2: Many thanks to traveling_pilot for sharing his selfie with Keanu on instagram. Photo was posted today and you can see trailers in the background.
Update X3: Further down in the comments traveling_pilot was asked if the photo is from today and he said yes.