- Electron is for desktop apps, so not useful for you.
- Flutter has better performance, but Ionic is decent as well, I would use Ionic unless you're making a game or graphic-intensive app.
- Ionic is more flexible since you have the whole NPM ecosystem available, while flutter is more recent, thus libraries for it are less in quantity and battle-tested than the ones in NPM.
- Ionic 4 introduced CSS variables, which improved immensely the theming process for the app, which was the hardest issue Ionic development had.
- Ionic has extended to many frameworks so it's compatible with Angular and React frameworks, meaning more flexibility, personally I would recommend Ionic with Angular over React since it's more suited to enterprise-level apps.
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One more, Flutter web support is indeed in Beta as Alejandro Ulate Fallas points out, you shouldn't use Beta features for production-level apps unless you have already tested such features before and you know those are sufficiently refined and battle-tested, if you need web as a hard requirement, and given you don't know much of flutter yet, go with Ionic, everything in Ionic is natively a web feature, because in basic terms what Ionic does in mobile is create a web view and embed your code in it.