Did Abraham, David and Solomon think having many wives was sin? Did Abraham, David and Solomon think having many wives and concubines was sin?
There were specific instructions for the king of Israel to not acquire many wives for himself, since that could have the effect of turning his heart away from the Lord.  This text predates the first king of Israel, so I would say that David and Solomon should have known that this was forbidden.  

(Note:  the Bible did not actually forbid them from polygamy, but from having a whole bunch of wives.)


  "When you come to the land that the LORD your God is giving you, and
  you possess it and dwell in it and then say, 'I will set a king over
  me, like all the nations that are around me,' you may indeed set a
  king over you whom the LORD your God will choose. One from among your
  brothers you shall set as king over you. You may not put a foreigner
  over you, who is not your brother. Only he must not acquire many
  horses for himself or cause the people to return to Egypt in order to
  acquire many horses, since the LORD has said to you, 'You shall never
  return that way again.' And he shall not acquire many wives for
  himself, lest his heart turn away, nor shall he acquire for himself
  excessive silver and gold.  Deuteronomy 17:14-17 ESV


As for Abraham specifically, aside from the affair with Hagar, he stayed married to one wife (Sarah) until she passed, after which he married Keturah and had children with her.

The Bible does not explicitly state that polygamy was forbidden, yet it would have been instructive to note that God created Adam and Eve--not Adam and Eve and Shirley and Tina and...