Are journal papers published in journals more useful than arXiv papers when applying for a postdoc? I am a PhD student in applied probability looking for postdoc jobs after submitting my thesis. What is the value of arXiv papers (not yet published in a journal) compared to papers published in a journal while applying for postdoc? For example, if I have two journal papers and one arXiv paper, whereas someone else has three journal papers, will he gets any benefit over me in applying?

My arxiv paper got rejected from a very top-tier journal after almost two years of review. I am working on the reviewer's comments; then I will submit to a new journal.
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.