If you only want to watch shows you will be happy with in the end, you should wait to watch anything until it finishes. Even after 3 episodes, critical opinion can change dramatically (see Erased). Even then you can't totally avoid disappointment, as other people may recommend something that you find poor (Soma for me). In the end, the only thing you can do is either not watch anything or be prepared for your expectations to possibly not be met.
Weekly airing shows fit my daily schedule really well so I'd rather watch as they air than wait until they end.
I don't believe there's some sort of guarantee of its critical/entertainment standing till the end once 3 episodes air, just more likely to be good. This is just one of the factors I'm considering.
the text I made bold: that's exactly what happened
I've chosen worse, not my first disappointment; I just don't like to drop a show - I've come so far, might as well finish it. It's damaging to me, I realize that but it bothers me in a different way if I do drop it.