The yellow line is Shark's Fin Arete/variation as described in current guidbook. Yardi Yardi Yardi is right of the right hand arete.

The guide describes the variant on the left wall. Climbing the pockets is harder than HVS and also no mention of a sizeable loose block.

Further; “ Gain the wall just in from the arete and follow the crack to gain the arete. Climb this in a fine position to the top. The arete gained direct is HVS 4c. © Rockfax” also I worked out this as climbing straight up on the left of the arete i.e. not starting around the other side.

The red line is Yardi Yardi Yardi (The line in the Moelwynion guide is incorrect). However, having top-roped the line of Yardi Yardi Yardi it does finish a bit more direct than the red line as you definitely use the crack where the yellow line is for your left hand (that runs vertically from the obvious horizontal crack). It then gets very dirty and feels eliminate as the arete is easily within reach. And its definitely E3 5c. I'm not sure where the direct start to the Shark's Fin goes, I would think it can go either side of the arete but quickly ends up back on the left hand side.