No you can’t. One way or another your request from your browser that have to reach (let’s say) mastodon will have to go through the web server then to the mastodon backend. Your request reaches domain.tld and your reverseproxy will send the traffic to the corresponding service… But you have 3 services answering to domain.tld. You won’t be able to get anywhere, or maybe to the first service where your reverse proxy matches but nowhere else. I don’t even know if a reverse proxy will allow to start with that kind of configuration. You’ll have to use subdomains, you’re not even (always) allowed to use subdirs (like domain.tld/lemmy) sometimes.