In my experience mushrooms visuals tend to be centered on distortion of objects and size, as well as some other weird shit. It also often seems to me that whatever I'm focused on while on shrooms is really big, and everything else in my visual field kinda shrinks to accomodate it. I generally have found shrooms to be a more mental and less visual high then mescaline or acid. Many people find this to not be the case though. I've never done more then 4 grams of shrooms at once, and that was quite a visual trip, very alien, but still not as visual as I've usually found mescaline or acid to be.
Mescaline has always felt like a super clean, incredably strongly hallucinogenic cousin of LSD to me. The visuals tend to be much more vivid then with shrooms, and they have this impossible to describe using normal language sparkly, colorful aspect. I've always found mescaline to be an extremly warm, gentle and accomadating drug whilst still being very strong. Mescaline is probably my favorite out of these three, but that kinda depends on my mood. It's incredably visual nature has always struck me as interesting because I've never done another phenethylamine that produced such strong visuals (I've not done very many of them though, and I expect that the 2C-Tx drugs produce incredable visuals as well).
LSD is probably the hardest for me to describe out of the three. I can visualize what it does (visually), but it's very difficult for me to put it into words. Everything always has struck me as shifting and deep and constantly moving on acid. It's also probably the most profoundly synethetic drug I've done, every sense always seems to blend in an amazing and hard to comprehend way.
In terms of fractals and morphing I find that mescaline produces a stronger fractal feeling (a general feeling, including, but not limited to, the visuals), while LSD causes stronger morphing. Mescaline often has completly removed me from the normal world and put me somewhere else entirely, a sort of total detachment from the existence I usually know and an introduction into a world of all things. I can totally see why so many native cultures have used it as a sacrament, it has, more then anything else I've done, shown me the "spirit world" so to speak.
LSD is impossible for me to describe. It's outside of my comprehension in all ways. I'm not saying it's stronger, it's just incredably hard for me to put into words what it does. All three of them are wonderful though, I can say that for sure, although I'm currently staying away from acid, quite possibly for several years, in order to protect my sanity (most people shouldn't worry about this, but it came to my attention about a week after the last time I did acid that I was losing my mind). I've never found mushrooms or mescaline to be trying on my sanity, especially mescaline. Even more then pharmahuasca and DMT mescaline strikes me as a true shaman's drug, amazing stuff.