Thumbs up for Brian's "Sweet Tone O' Slash" preset; I was messing around with some Slashy presets, but couldn't achieve anything much better than that one
so I haven't taken it further or posted my experiments.
That said, there is a fair old pile of information out there about Slash's gear and settings -- perhaps too much, actually
since it seems like Slash has used subtly different stuff and settings through the years, and it can be difficult to figure out what's what with info scraped off different Web sites!
Slash's Appetite for Destruction
tone was famously produced on a rented, modded Marshall "metal panel" amp (technically, it would seem, a 100W Marshall Super Lead Model 1959 with an added extra pre-amp tube to provide an extra gain stage, as well as other switcheroos with particular capacitor types, etc.; the recent Marshall AFD100 attempts to recreate that kit). So the British S100 model in AT3 would obviously be a reasonable starting point, but I am entirely unsure about how one would attempt to mimic the extra gain stage. There's no real way to imitate that in AT3 other than the traditional but unauthentic approach of slapping an overdrive pedal in the front.
Otherwise, Slash is strongly associated with Marshall Silver Jubilee amps, which seems to me to be not un-JCM800ish in many respects (and I think he has sometimes used fairly straight-up JCM800s, at least for recording), and so the British Tube Lead 1 is clearly a good starting point on this front. Generally Slash uses EL-34s in the power amp, though he has used a separate amp with KT88s for clean tones; at some point he also used 6550 power amp tubes in some sort of Marshall for some sort of purpose. There are no KT88 or 6550 power amp options in AT3, but one might try messing around with the 6L6 options, perhaps.
Depending on the amp and the era, Slash's settings seem to vary (and, on tour, they probably a vary a bit night to night, tweaking for the venue), but his general starting settings for everything
seem to me to run in the 6-8 range, with mids and/or treble sometimes pulled back towards 4-5, and bass and/or presence bumped up towards 8 (though I've seen photos of the AFD100 with its presence back towards 4-5 ... though the AFD100 may be a different beast anyway). Master volumes seem to run from 6ish to all the way up (going for that good ol' power tube distortion), while pre-amp gain seems to run (again) more usually in the 6ish vicinity.
Slash doesn't seem to use a lot of pedals. Famously, he boosts for leads with an EQ pedal set to "frown", bumping up the mids (fairly readily replicated with either the of the EQ pedal options in AT3) and a delay pedal set for a slap delay for solos (again, fairly readily replicated with probably whatever delay pedal in AT3).
The key thing is probably playing with the amp settings a bit, and picking some appropriate cabs. (Slash uses has used Marshall 1960A/B 4x12s with 70w Celestion Vintage speakers, but also cabs with 25w Greenbacks, and also cabs with Vintage 30s; not always sure quite which cab models in AT3 will get closest; some of those speaker options at least don't exist.) But for any effort in zeroing in on that "intro to Sweet Child" tone, trying to trick the British S100 into thinking it has an extra gain stage remains perhaps the puzzle that causes me the greatest uncertainty about how to approach it.
I got some good results by pairing a British S100 model with a British Tube Lead 1 model, juggling "Slash-like" settings on each, putting overdrive and/or EQ pedals in front of one or both, and trying different cab options that seemed vaguely appropriate (Marshally type things), but by "good results" I mean more that I got some pretty decent heavy blues/rock tones rather than anything that I would say distinctively seemed like any given Slash tone.