Also its helpful to understand what particles are, not only in Sins but in many other games. A particle is usually a simple sprite, meaning its a 2D picture - which is then instanced ("emitted") in relatively large numbers to produce a certain effect. A particle can have its color, size, orientation, transparency etc. changed during its lifetime, which in turn produces different effects.
Also, every particle usually has a lifetime, which is basically the amount of time it stays in the simulation and/or is displayed on screen. This is also an important factor in determining the final look of a particle effect.
And finally, every particle needs an emitter. Emitters are geometrical objects whose surface or volume is used to generate particles in a specific way. A sphere will generate particles in a different way than a flat surface, for example.
So to answer your original question, to create a 3D particle simulation, you do not need to make a 3D rendered footage (if I understood you correctly) - you need to create small 2D images which will then be used in a 3D particle simulation (using emitters like a sphere for example) to achieve a look of a 3D effect.
Usually in the Particle Forge you will have more than one particle simulation going, meaning more than one emitter, particle sprite or affector (these are parameters which influence particles and change their properties during simulation).
