my opinion
It doesn't take Deca/Enan three weeks to "become effective", even though it might feel that way. The second they begin to release into your bloodstream/attatch to receptors, they start exerting their androgenic and anabolic effects.
With prop's short ester, most of the test injected is (or has been) in circulation for 3-4 days. With Enanthate or Deca, only half of the test/nandralone is slowly released over the period of 8-14 days. So with prop you can build up the active test levels really fast by injecting every day or every other day and it is going full force within a week. But Enanthate/deca,etc. (long-esters) have only released HALF of their active hormone after that week or sometimes only a third (half-lives are often longer than a week).
So if you inject 200mg test prop on Monday, then again on Wed and Fri (or 600mg/week), by sunday you'll have experienced the effects of nearly 600mg "active" running around your body and you can feel it by then. IME, I can't feel prop until the first week has gone by. Its individual.
OTOH, if you inject 600mg test enanthate on Mon, by sunday only around 200-300mg has been released. The other 300mg is bound to its ester and will slowly be released over the next week or two. Within 3 to 4 weeks (due to all the half-live degradations) you'll have a high, stable level of test in your blood and this is when you notice it.
This is how I understand it, maybe someone knows better and can share.