How do I pose a hypothesis for a partial correlation?

Hello everyone,
in my thesis I have following hypothesis:
3a: Internet skills are positively correlated with the frequency of experiencing risks online
3b: The positive correlation between internet skills and the frequency of experiencing risks online is at least partly caused by the amount of time spent on the internet on a daily basis.
For 3a, I calculated a Pearson correlation, and for 3b a partial correlation.
My supervisor told me that I'd have to calculate a mediation for 3b though, or rephrase the hypothesis. He did not have a suggestion though.
I don't really want to do a mediation, because I already have two mediations in my thesis and I don't think it fits the theoretic idea behind the hypothesis .. so I'd be thankful if someone has a suggestion for me how to phrase the hypothesis for a partial correlation! :)