There is no one app that runs on all of those right now. Snikket has apps for both Android and iOS but it's two apps not a cross-platform app. You'll likely get best results using an app optimized for the platform in question (such as Cheogram Android, Snikket iOS, beagle.im, or Movim).
Which platforms do you want it to "cross" ?
Specifically Android, iOS, and macOS/browser
There is no one app that runs on all of those right now. Snikket has apps for both Android and iOS but it's two apps not a cross-platform app. You'll likely get best results using an app optimized for the platform in question (such as Cheogram Android, Snikket iOS, beagle.im, or Movim).
Are those as modern as it gets? They all look very outdated UI-wise.
Well, perhaps the UI themes don't look like what you prefer, I can't predict that one way or the other, but none of them are "outdated".