Re: Star Trek III - The [Speculation Begins] (Spoilers)
TMP was number 1 for at least the first 4 weeks of its release.
BoxOffice Mojo only had the first three weeks of TMP listed, so you may well be right. But the point stands that only two Trek films spent multiple weeks at number one.
"I had no idea you were so... formidable. " - Anan 7 to James T. Kirk, A Taste of Armageddon