I have never seen a number below 3% either. But for sure it must be possible. I zone quite small areas at a time so maybe that means I get less mass retirement and deaths.
I still think the 3% figure is a bit of a false figure... or we don't have the information to fully analyse it. With 3% unemployment you may still find you have jobs free. that's maybe because you have some educated jobs that aren't being filled but you don't have the staff to fill the vacancies... as an example. Then it's the lower educated people that you need more jobs for, but the figures don't have enough detail to tell you.
I actually think the best strategy is to have everyone fully educated to 100% and then make sure that not everyone is employed. That's the only way to make sure all the jobs are actually filled. You just need to make sure that you have enough people to fill the jobs.
I also agree with Shiggs that when you put in education you will lose a proportion of your work force to them going to school suddenly.