About Harrowing, the harrowing skins have already been made, Ravenborn Leblanc and Underworld Wukong.
About Zac, they are having great difficulty making a skin for him right now because if you were to put clothes or any definite-shaped objects on zac's model like a hat, etc. it would totally screw with his rigging and there would be many problems with how those clothes/objects would act when zac used his abilities that change his anatomy (turning into a puddle after his jump, turning into a giant ball blob-y thing for his ult).
About Jinx, I'm guessing she very well could get a skin soon, there are only two new skins on the pbe cycle (Reaper Soraka and Victorious Morgana) that started today and sometimes they put in additional skins right after the first new pbe cycle patch, so really idk.