Particle effects work differently than in Quake 3 but the same as Tremulous. Please see the Tremulous documentation (pdf) for information on creating particle effects for now.
Particle format is game-specific (mods may modify it).
There is a particle effects editor for Quake 3 that will likely need much modification to work with the Tremulous-style particle effects system. Source code is available from GitHub. A very brief tutorial on its usage by Frozen Sand (creators of Urban Terror) is also available.
An older particle effects editor (not updated in many years) was also available. Dead link: