The A1000 was the first offering, followed about 2 years later by the A2000 and A500. Being the first iteration, the A1000 had many quirks and suffered from a stylish but impractically slim case size (for the era). The A2000 addressed the lack of expandability, while the A500 answered the low end of the market. Though the CPU did not change, there were a lot of changes in the overall chipset -- one large one being that the A2000 came with 1MB of chipset (dedicated) memory to the A1000s initial 256kB.
The A3000 came another ~2 years later -- was a little late to the party -- and delivered in a number of areas, but perhaps tellingly, many professionals would stick with the A2000 + 68030 accelerator boards. Accelerators from the leading company GVP were stable and much faster than initial A3000s, beyond which many video/CGI orientated cards would not initially fit in the A3000. That people moved the A3000 hardware to third-party cases is perhaps saying a lot about expandibility vs sexy cases.
The models that were pretty pointless were the half-way (or less) upgrades -- the A2500 and A1500.
If it wasn't for the third-party hardware developers, the Amiga would have died much sooner and the A2000 was the workhorse for these companies.