All three use the same basic SFZ engine.
Z3TA was the first of Rene's synths - Cakewalk bought RGC out and Z3Ta w/ him. As mump sez it is an Virtual Analog synth. Most of the "samples" are analog waves. It has a waveshaper to change the actual waveshape and, in Z3TA + 2 you can sync those changes, thus affecting the sound of the sound before any envelopes and filters and such.
Dimension was Rene's next synth, made for P5. It was more of a sampler, with a large sample pool of acoustic instruments. It is still a synth, tho, with filters, LFOs etc. And it can be layered via elements (each of those a single mono-timbrel synth in itself). The patches/presets were mostly of acoustic instruments, but there were some pads and spacey sounds too. DimPro mostly added more samples - it went from 3 gigs or so to 7. And more presets. And was sold as a stand alone synth before it got included w/ SONAR.
Rapture has a different set of sampes/SFZ, 6 elements and step sequencers. All the organs that were missing in Dimpro (which had 10-20) were in Rapture, as well as more electric sounds - synths like moogs & arps, more eps, etc. However, Rapture can't play back samples but plays a "wavetable" approximation of the sample, giving Rene's SFZ the clean playback such are known for, including little aliasing. DimPro will wavetable samples less than 3000 samples long (I think that is right), but will play longer samples back in all their sample length glory.
Do you need all three? No, not if you have all the DimPro samples for Rapture (since the wavetable will do a good job on them). Most patches don't use all 6 elements, but DimPro can't emulate the step sequencers, so Rapture is the most flexible of the two. But it can't exactly replicate longer samples, either.
I divide them up according to what I'm trying to do. I use Z3TA for a VA, doing modular-style stuff w/ it. DimPro is for mostly acoustic instruments. Rapture is for synth stuff, but it cuts better than Z3TA (or Alchemy, which is my go-to VA). The basses, for example, even if they are synth based, still sound less synthy than Z3TA. Z3TA is better for buzzy sounds.
Ideally, there should be one synth using SFZ, with all the Z3TA synth oscillator waves, Rapture's various short wavetables and longer, DimPro samples. But Cake serialized them and made money from each as they added to the sample pool. That is where the synth money is - in libraries. Cake just used a different synth to sell them, rather than nickel and diming the customers.
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