Particle effects work differently than in Quake 3 but the same as Tremulous. Please see the Tremulous documentation for information on creating particle effects for now.
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) is also available.