Vent a portable air conditioner with a 6 inch hose through a 3 inch hole in the wall

You could build a wood frame that fits in the window with an interior hole large enough to accommodate the AC unit. This also has the advantage that you won't have to hold the window open, while juggling the unit into place. You can install the light weight frame, then slide the unit into place.

I built one when I lived on the second floor of an apartment building, because I was always afraid I would drop the unit out the window while installing it. I used foam tape around the exterior and interior of the frame, to make sure I got a good seal around the window and AC unit.

For a unit that only vents through the window (not sits in it), you could build a smaller frame with a port to allow the vent to pass through. Filling the interior of the frame with Lexan will reduce the amount of wasted window space.

(You could always use smaller lumber for your frame, if you want it to have a lower profile. The frame size in the image is just an example, to illustrate the idea).

The specs on the AC unit say it's 14.2" wide, so it will certainly fit in the opening. You don't say how high the window is, but if it's a double hung, two pane, slide-up window like the ones on the building across the way, you might have to remove the window to make this work then fit a glass or plexiglass pane into the space above the air conditioner after it's mounted. And be sure not to drop that thing in the street... ;-)

EDIT: Typically, units like this rely on the top sliding pane of the window to hold them in the opening. The AC unit is heavy, and it will want to tip backwards out of the window. You can see the tab/rail along the top of the AC unit in the picture just below. If you drop the sash down behind that, it will hold the AC unit in place, as it can't topple out of the window.

So; typically, you would pop the front cover off the AC unit, then screw the bottom of the unit to the window sill so it can't slide forward out of the opening. Then drop the sash behind that rail and screw it to the sash. Otherwise, it would be possible for the sash to move up and allow the air conditioner to topple out of the window, which would ruin the air conditioner and could also maim or kill somebody below.

In your case, if the unit won't fit in the bottom pane and you have to remove the window, you'll need to secure (and secure it well) some kind of support across the window opening to replace the window sash you removed, so the AC unit can rest against it. And you'll still screw that rail to the cross support to ensure that everything stays in place.