* There doesn't seem to be much progress on an official iOS app, but it is something that users are asking for.[1] An unofficial closed source app called Möbius Sync does exist for iOS.[2]
* S3 and Dropbox are not officially supported,[3] but you can always sync one of your Syncthing nodes to cloud storage on your own.[4]
[1] https://github.com/syncthing/syncthing/issues/102
[2] https://www.mobiussync.com
[3] https://forum.syncthing.net/t/how-to-sync-files-with-aws-s3-...