Well there was some 2MB 72 pin modules around back in the day. I guess for systems that came with 2MB on the board, then you could upgrade with a 2MB stick. I don't know really the rationale behind them existing. The reason I had one is that I had an A1200 as my "real"computer (daily driver) in '93 and the magazines were saying that if you got an accelerator, the memory map was such that more than an additional 2MB would interfere with the PCMCIA slot, but it also seemed that 2MB SIMMs were rare and hard to get hold of for the Amiga accelerator suppliers. So when at a computer fair, I found a used 2MB dirt cheap, because to most of the PC world it was a stupid size, I bought it so I would have it for a future accelerator buy. I didn't see much point in the '030 accelerators though and was waiting for '040s or better, and meanwhile my A1200 seemed fast enough for what I was doing. So a couple of years passed before I got an accelerator. However, during that time I started screwing around with PCs, because these boring old business boxes seemed to be getting more important, and more fun. So the 2MB was underemployed on the Amiga front, so various thrown together from used parts PC builds borrowed it. Turned out though that accelerator manufacturers managed to remap around the PCMCIA memory addresses and you could use 4, 8, 16MB no problem in later accelerators.
Don't think I ever ran 3MB, but 1MB on board was common for late 286, so adding two 1MB SIMMs to that would do it.
Unicorn herding operations are proceeding, but all the totes of hens teeth and barrels of rocking horse poop give them plenty of hiding spots.