I don't think sub 18 5k or sub 38 10k are needed for sub 3 marathon. 18:30/38:30 are plenty. Same goes for half marathon. If you are a good aerobic runner then you can hit 1:26:xx or even low 1:27 and still run sub 3 marathon.
You will need to hit long tempo runs. If you can run 9 miles non-stop (no water break) at 6:30-6:35 pace and feel like you ran at 85-90%, that's more important. You should also increase your mileage to 65-70 for at least 4 weeks before you taper. Mileage bump before taper helps a lot in my opinion.