I am thinking of creating an app that will allow you to contribute to MusicBrainz, Discogs, and Wikidata at the same time along with easily allowing you to create external links to various music streaming sites like Spotify and YouTube.
Does one already exist that does some of this in some capacity?
Doubtful. Discogs and MusicBrainz have incompatible guidelines (e.g., Japanese) and practices (e.g., Discogs doesn’t separate Nippon Columbia from Sony Music’s Columbia, nor do they separate Decca Classics from Decca Records, and so on…).
I created a library to add data to Musicbrainz and a library for adding data to Discogs (not published, but I used it successfully).
I haven’t used them in a year or so, so it’s possible/likely that they need to be tweaked to work with the current versions of the websites. I’ll probably get around to that sometime this year.
Since I’m a 20th-century throwback, I wrote them in C with an eye to writing desktop and command-line apps, and wrappers for languages such as ruby/python/perl.
If you’re looking to do a web or mobile app, I’m not sure how much use they’d be for you.