Could your own preprints of the same paper on arXiv and other sites be considered as plagiarism when the journal checks this paper using, for example, Turnitin plagiarism program?
I suppose that an automated system might flag it, but a person would certainly recognize what happened.
However, note that some journals won't accept a paper that has appeared on arXiv or elsewhere prior to submission. Some will place a copy there after acceptance, but they want to control access themselves.
But in any case, it isn't plagiarism.