I can't tell from the picture if that triangle is recessed within the surround, or projects.
If it's the latter it wouldn't be hard to make a spanner with a bit of flat steel, drill a hole and file it into a triangle.
If it's recessed, maybe shape the end of a bit of round tube of appropriate size, around a triangular file.
If it’s a male head a street lighting key should fit…I have various sizes. If it’s female then a couple of appropriately sized screwdrivers percussively fitted will work
But obviously you would actually need a pair of these...?
To allow for both Left-handed and Right-handed applications....!
A useful tip is to wrap a bit of PVC insulation tape around the handle of the Right-Handed hammer..
so you don't accidentally pick up the Left-handed one for the wrong usage!!
It is never good to whack a right-handed fixing with a left-handed hammer.. As that would be just asking for trouble!!
But there aren't many problems that a good quality pair of Left & Right handed hammers can't fix!!
So if all else fails remember the guidance from our esteemed member Mort2376..!
And do not forget to multiply all comments in this post by the Friday-Night-Beer-Correction-Factor to discard any waffle and/or tosh!!! (If there is any?)
"Tri-Head" is possibly my only useful contribution!?