Did Abraham, David and Solomon think having many wives and concubines was sin?
Having multiple wives was not a sin back then, as it is not a sin now (outside if it being illegal).
At the time of Abraham, David, Solomon, and Jesus, polygamy was part of the culture.
Even in Jesus' times, polygamy was allowed and part of the customs of the Israeli people. The historian Josephus noted that Herod was allowed to have multiple wives, because it was the custom in Israel at that time.
Furthermore, it was allowed per the Bible. When God gave the commands to Moses, he gave special instructions for the situations of multiple wives:
Had God detested polygamy, there would have been a command against polygamy. By comparison, God said that Kings shouldn't take too many wives so that his heart isn't lead astray, which is a further command showing that polygamy was allowed.
Historical and biblical evidence shows that polygamy was common practice and acceptable in the site of God and the people during the time of Abraham, David, and Solomon. Their polygamy was not a sin, just as it is not now..
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.)
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...