He couldnt play the new ball so there was no way he would had survived at no.3 . I get the point tht the collapse meant he would still come, but it was not his job, and had difficulties seeing off the new ball
As a batter he was very limited, so he had to play within those limits which is why either lower order or middle order was his place. But not top order.
Thing is while the likes of ahmed shezad, nasir jamshed, and younis khan sucked at the top order, guys like sohaib maqsood and umar akmal were also no good at the middle and lower order aswell.
So misbah was like the bridge between these two collapses and he had to do two things. Make sure the fall of wickets stop from one end and than at the same time also score runs. If he would had batted at no.3, the fall of wickets from one end may have stopped a wicket eaelier but than his scoring rate would had further reduced.
I remember, things became soo bad that when Imran Farhat ended up scoring against dale steyn as an opener, misbah decided to keep him in the odi team because the rest were so bad atleast he got runs off the kighty saffars bowling attack