The current Technical Specifications for the iPhone SE (3rd generation) say that (emphasis mine)
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Syncing to a Mac or PC requires:
- macOS Catalina 10.15 or later using the Finder
- macOS High Sierra 10.13 through macOS Mojave 10.14.6 using iTunes 12.8 or later
- Windows 10 or later using iTunes 12.12.10 or later (free download from apple.com/itunes/download)
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So while Mojave is no longer one of "the most recent three", and is not getting any love from the likes of Adobe and Microsoft, it sounds like you should be able to load music onto your iPhone using iTunes.
It is possible that you might encounter problems.
I know that when iOS 17 came out, Apple briefly referred to using High Sierra with iTunes 12.9 or later. There was no iTunes 12.9 for High Sierra. Later, Apple changed the requirement back to iTunes 12.8 (and there is a 12.8.3 for High Sierra). People who tried using High Sierra and iTunes 12.8.* to manage iPhones running iOS 17 sometimes reported success; other times reported resorting to workarounds (like extracting updates from Xcode!!!) that should not have been necessary.
But Mojave supposedly is enough to manage an iPhone SE (3rd generation). Try it, see if it works, and if it doesn't, you may want to make use of whatever free telephone technical support period that came with that iPhone.
