a three year old iPad is definitely not as useful as a three year old laptop. This is the case, simply because of iOS updates. a three year old iPad is ridiculously slow. Not surprised by this at all.
I'd say a 3 year old iPad isn't ridiculously slow - maybe a 4/5 year old one, but not 3 year old. However, I do agree with what you say. Laptops have a longer lifespan. You could probably get through the whole of high school with one laptop.. an iPad you'd be stretching things wayyyy too thin.