Telegram would connect to a project subdomain like hello.glitch.me on say port 443. But hello.glitch.me doesn’t resolve to an IP address belonging to that project’s container. It resolves to a proxy server, and that proxy server receives the HTTP request. That proxy server connects to the project container on port 3000 and sends a copy of the request there.
So as far as Telegram is concerned, it is connecting to port 443. And as far as your project code is concerned, it’s receiving requests on port 3000. These should mean that overall it might be able to work, but…
I would have to take your word for it that it still doesn’t work. There are many other fiddly things about Glitch that could break it. Notably (i) there’s the system where it sometimes delivers a “Waking up” screen when your project container isn’t running, which has been known not to be friendly for anything other than a human sitting at a web browser, and (ii) they’re doing some not-publicly-detailed filtering to block abusive requests, which has been known to be disruptive for certain non-abusive integrations (1) (2).