How do instruments in Mainstage 3 compare to Mainstage 2?

The evb3 has been updated and renamed to Vintage B3. It received new distortion modes , which sounds more like a Leslie tube amp. And the Rotor cabinet has been revised. Very good simulation. I compared it to real Leslie recordings and you can just drop it it, difference is very small. Key click is more clicky. This alone makes it worth upgrading,

The EVP88, which is called Vintage Epiano, got a few new models, which are more realistic.

The other pianos are sample based. Just download all the content, that comes with Mainstage. My experience is, that if you tweak the velocity mapping a bit, the pianos are much more playable. Therefore, you can use those MIDI plug-ins, which allow velocity processing and various other things.

DaCaptain

The acoustic pianos in Mainstage 3 I believe are basically the same sample set as in MS2, but they're EQ'd and processed better. If looking for better pianos, there are many acoustic piano third party virtual instruments to choose from that you can use within MainStage. The electric pianos in MS3 are much improved. In MS2, the EP's retriggered notes with increasing velocity each time the same key was hit when the sustain pedal was pressed, causing an annoying and unnatural response. That has been fixed in MS3.

Other new features are MIDI FX plugins in the channel strip, including a very cool arpeggiator and and MIDI Scripter. A new plugin, Retro Synth, is also an excellent addition.

I use Synthogy Ivory II(about $329). It has rich, detailed Steinway, Bosendorfer, and Yamaha grand samples. Also, you might consider the Ivory II American Steinway D($189). I'm hearing great things about VI Labs' Ravenscroft 275($199).

Velocity mapping really helps playability and response for any pianos. A few photos will illustrate it.

Select a channel strip, then the MIDI tab in the screen control inspector>velocity input. In the upper left there are preset velocity curves than can be edited extensively. Fig. 2 shows a preset that was edited by dragging areas of the curve and clicking on multiple points.