Most gaming related communities nowadays are on Discord, which makes it harder to find up-to-date information on topics like quake 3 mapping online.
Discord actually has a great 'search' function that makes it easy to find info on a specific topic, however, you won't find it through google like you would info in this forum for example.
So here is a list of some active (and some less active) Discord communities with mapping sections for quake 3 and other id Tech 3 games:
- Defrag
https://discord.gg/ZG4dKNVQJu
This is the Defrag (a Quake 3 mod) discord which has one of the most active #mapping channels and always someone willing to help with any questions. - JKCommunity
https://discord.gg/gFKwH85
The Jedi Knights community servers. Lots of innovation coming out of here. - Xonotic
https://discord.gg/0uxcsw1XMkaOB936
I'm not really familiar with Xonotic, but they do use Radiant and their #mapping channel seems to be quite active. - Quake Mapping
https://discord.gg/j5xh8QT
Mainly focused on Quake 1, but also has a sections for other games. - Level Design
https://discord.gg/f4UV7We
A server for discussing and learning about level design. Also has several channels for different games, including Quake 3. - Blendiant Task Force
https://discord.gg/2hyq8re
This is the home of the BSP Importer and MD3 exporter plugin for Blender - Defrag (old/archive)
https://discord.gg/UqQEc6h
This is the *old* Defrag discord which was abandoned in order to create the new one with moderation. This can still be useful for archival purposes. You might find your question answered by using the search function in here. - CPMA
https://discord.me/cpma
CPMA is a popular Quake 3 mod. Although not very active anymore, this server might have a bunch of useful info archived. - Custom Map Makers (Urban Terror)
https://discord.gg/t48GHkA
Urban Terror mapping community - TheWolfTeam
https://discord.gg/eVhQqA2
Small Wolfenstein Enemy Territory community
This list is also part of my id Tech 3 Mapping Trello, an actively worked on collection of resources, guides, tools, etc.