There was an addon for pi-hole but its been deprecated for some time since the adguard addon is better and easier to use. Of course using an rpi3 with 1gb of RAM (can’t emphasize this enough) for both your home automation server and the DNS server for your LAN seems like a really terrible idea. But if you want to slow performance on your network to a crawl then go for it.
There’s a plex addon? What media service are you referring to?
Although seriously putting this on an rpi3 too is really not going to end well.