I want to deploy an application with sqlite3 as the database on Heroku. However, it seems to be that Heroku doesn't support applications with sqlite3 as the database. Is it true? Is there no way to deploy my sqlite3-backed application on Heroku?
As Heroku's dynos don't have a filesystem that persists across deploys, a file-based database like SQLite3 isn't going to be suitable. It's a great DB for development/quick prototypes, though.
Heroku do have a Postgres offering however that will suit - with a free tier and a basic $9/month tier that are good for hobby/small projects. The biggest benefit over SQLite is that you get backups that you wouldn't get otherwise (plus all the other Postgres features).