I am not 100% on this but looking at the photo I would suggest the three metal parts protruding at the very bottom of the fitting unscrew.
Have dealt with lots of similar fittings and generally thy are held on as I stated. Try either a pair of pliers and gently but firmly try unscrewing ANTICLOCKWISE. Or try donning some rubber gloves to get better purchase on them.
Also, they should not require much force to remove them as suggested, so if they are not budging I would look elsewhere on the fitting to see if they undo differently.
If trying pliers be careful as the pliers may slip and scratch the three 'heads'. Soooo put some cloth or rubber between pliers and the 'heads' before gripping them.
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