It's true that 3-mmc is far less smelly than 4-mmc aka Mephedrone. Apart from that, they smell completely different:
As for 3-mmc, the closest thing coming to my mind is a certain type of medical plaster (German brand name "Leukoplast"
)
Regarding the "cat piss" smell of 4-mmc:
I've been using meph since 2009 and been living with cats since even longer ago, and tbh to me cat piss is an analogy I'd have never come up with...
Genuine pre-ban meph (shiny white sugar-like crystals) to the biggest part had that smell certain railway tickets or bank statements have, when they come fresh out of the printer (not a home or office printer, speaking of public atm/ticket terminals here).
Then add a slight breeze of chlorine tabs (for swimming pools) and you are quite there!
...probably some residues of precursors or re-Xing solvents (which I can't specify), because after doing a proper recrystallization there is hardly any smell left at all.
As for all the post-ban meph I've come to sample since, the smell I described above most of the time has been overshadowed by aggressive drain cleaner type of stenches, plus residues of whichever solvent had been used in the synth, plus that tearjerkingly bitter/peppery bromine sting :/
