Sounds good. Under "acousmatic concert" I imagine "Musique concrète", more or less, yes? Does that mean you work with recorded samples?
The setup depends of course on whether you also need sound input (I guess not in that case), and whether your speakers are active or passive (if they're passive, you'd also need an amp). You don't actually need a mixer, since you can do all the mixing in Max, but sometimes it can feel a bit nicer to control these things outside the computer, analog. I like using mixers as well.
Now as for "multi-channel diffusion" and suggestions on what objects to use... You should be a little more specific. What exactly do you want your patch to do?
If you want to do the diffusion with the mixer, then that sounds to me as if you don't actually need the patch to do any diffusion at all? I.e. do you plan to just go out of your computer/interface in three channels, each containing one soundfile, and then use the mixer to route these channels to 8 speakers? In that case, your required patch would probably be very simple.
Or do you still want to create 8 channels out of your three soundfiles within Max?
As for seeing the waveforms: You can use [scope~] to see the -current- waveform as it is played back, but if you want to see the waveform of the whole soundfile, you'll probably have to load it into a [buffer~] (with a (read) message) and play it back with an object like [play~]. You can then display the buffer's waveform with the [waveform~] object. (Or simply by double-clicking on the [buffer~] object)