From what I can tell, science level 3 is superfluous. I had already solved all the puzzles before getting it, and all it does is increase the rate at which I accumulate tech points. Is there any actual purpose for level 3, 'cause right now it seems like it's just a waste of 150,000 tech points.

Nope, that's all it is good for is to help you gain tech points faster which if you have no other techs at all really comes in handy but if you are near the end and it is a choice between getting more exploration or more science, go for the exploration or something else that you really, really need!!

I like to buy Level 3 Science as soon as I can (just after L3 Farming) -- the tech boost is well worth it...it's a long wait to get 150,000 points, but you'll get the remaining techs much faster.

To check, I tried to get only one villager at my research table, and it seemed like a master scientist with level 3 science tech earned 30 tech points per research job, rather than 10 without any science tech...I couldn't check, but I'm guessing a master scientist with only level 2 science would earn 20 points.

If I'm right, you earn tech points 3x faster with level 3 science than you would if you didn't buy any science tech at all, and you earn tech points 1.5x faster if you buy level 3 science as soon as you can after level 2 science.

(I might be biased, though -- I like science )

I like to get science 3 early on after L3 farming as well, the rest seems to come a lot easier if you hold out for that one.