It's funny that you posted this forum, because I usually have tv land playing every night up till the wee hours of the morning and Threes Company allways comes on. I love to watch this show it is very funny still even after all these years.
Did Jack sleep with them, no at first he would have if they let him, but they didn't.
Then later when he probably could have he looked at them as just really good friends, and did not want to take advantage of them.
He may have wanted to be a play boy, but he had a little bit of old fashioned values in him.