So if you had gotten an iPad 2 on day 1, I feel like you would have had a reasonably long life on the device.
Your case with the iPad 3 is a little unique. It was the first device with a retina screen and was arguably underpowered for that task. This is why apple replaced the iPad 3 with the iPad 4 just 6 months later in the same year.
The iPad 1 and the iPad 3 are both underpowered for their typical workloads. The other iPads however have stood the test of time fairly well. My iPad Mini 1 (which is essentially an iPad 2) is a little slow, but serviceable and my wife's iPad 4 still works great.