Gotta respectfully disagree with you. First, with regards to Rush, they destroy Joni in terms of popularity. Just take a look at these album sales links from the RIAA. The first is Rush's, then Joni. No Contest. Joni does beat Rush in acclaim but nowhere near the margin they beat her in popularity.
Rush beat Joni in popularity but I think Joni takes influence and acclaim fairly easily. Anyway, objectively measuring a rather popular artist against a rather influential and acclaimed is not possible. So if you're giving popularity a little more relevance than for instance Brian does, it is not a problem as long you're being consistent with it, and I think you are.
With regards to the Band, I agreed its a bit closer (that's why they're 3 and Rush is 2). Joni may squeak by them in terms of popularity but I think the deciding factor is acclaim and influence. I think Allmusic's Bruce Eder sums it up better than me:
For roughly half a decade, from 1968 through 1975, the Band was one of the most popular and influential rock groups in the world, their music embraced by critics (and, to a somewhat lesser degree, the public) as seriously as the music of the Beatles and the Rolling Stones. Their albums were analyzed and reviewed as intensely as any records by their one-time employer and sometime mentor Bob Dylan. Although the Band retired from touring after The Last Waltz and disbanded several years later, their legacy thrived for decades, perpetuated by the bandmates' respective solo careers as well as the enduring strength of the Band's catalog.
Well, Eder is right that The Band is among the most influential and acclaimed artists of there era (of any era actually), but Joni Mitchell is as well. Her recorded output is at least as acclaimed as The Band's and her influence outdoes that of The Band. Joni was hugely influential on contemporary and alternative singer/songwriters, hugely popular styles since the 1970's. The Band set the stage for all the roots rock and alt-country bands to come, important as well, but not quite as big as the styles Joni influenced.
Let's also take a look at Rush vs. The Band:
Rush take popularity easily. Rush's influence is rather limited. When they broke through, prog was already an established genre. They had influence on later prog styles (neo-prog, prog-metal) but these are not very big styles overall. The Band takes influence fairly easily.
Rush's acclaim largely rests on a few albums from the late 1970's/early 1980's. These are acclaimed records within their respective style but they lack the widespread acclaim The Band had in the late 1960's/early 1970's. The Band also takes acclaim fairly easily.