Are journal papers published in journals more useful than arXiv papers when applying for a postdoc?
ArXiv papers are preprints, not publications, so their value in the context of a job application is definitely lower than a paper that has been accepted for publication in a peer-reviewed journal. In your example, assuming all other things being equal, the answer is that yes, the job candidate with 3 journal publications would be at an advantage relative to you with your 2 journal publications and 1 arXiv preprint.

Of course, all other things are never equal, and papers differ enormously in their quality, scope, and significance, so that it’s quite possible that your one arXiv preprint might still end up overshadowing any of the three journal publications of the other candidate. So I would advise you not to get too caught up in these sorts of numerical calculations. The number of papers has some significance, but it’s very far from the only thing people care about.