Nouns that don't require a measure word
The trick is the omission behind (again).
It is the objective that is omitted, not the MW words.

The complete words are: 
三年(时间),四周(时间), etc

As you can see now, there are MW words. Just too obvious to add the objective again.

Another example, after business dinner in CN, people usually ask 刚喝了几杯,  which means 几杯(酒). But we never say 几酒.