Stevie Ray Vaughan and Double Trouble. Double Trouble started out as drummer Chris Layton and bassist Tommy Shannon. They later added a keyboard player as a third member.
Ben Folds Five, as far as I know, only ever had three members.
Triumvirat, a German prog-rock band sometimes known as the German ELP, had three members until their bassist/vocalist left the band. They added both a bassist and a vocalist as replacements, bringing the count to four.
The Fab Four had a lot of “5th Beatles.”
10,000 Maniacs
One of my favorite album covers is the Chad Mitchell Trio, with four members walking across a college campus. Now, the fourth is Jim McGuinn* who played banjo with them, but it’s still jarring.
*later changed his name when he was given the “spirit name” of Roger, when he was dabbling with Eastern Mysticism while with the Byrds.
Last week there was a Pentatonix Christmas special where the group consisted of only four members.
I missed the beginning, so there might have been an explanation, and the group still lists five members.
Maybe they recorded it between the one guy leaving (amicably) and the new guy joining. That would be between September and October…
Timbuk3 started as a duo and later added two more members.
Both hip hop artists called Timbuktu are soloists. But there are “tu” of them.
I’m stretching the rules here (ok breaking them into little pieces)
3 Dog Night recorded their hit song “One”
Maroon 5 currently has seven members.
I’ll allow it, since Three Dog Night did not, as far as I know, have any actual dogs in the band. 
The Thompson Twins have had anywhere from 3 to 7 members at various times. None of whom were named Thompson, btw.
We Five started as a quartet before adding a fifth member.
I think it was George Thorogood and the Delaware Destroyers, but I remember seeing a photo of a band onstage with a sign “The world’s only five man trio”. There were four band members.
I believe the X-Factor band Fifth Harmony had a member quit… so now there’s only 4 of them.
The Four Horsemen had 5 original members, unfortunately 2 of them are dead so there’s only 3 members left. 
Wednesday 13 is both one guy and a band with fewer than 13 members.
Premonition 13 is a band with 4 members.
And Then There Were None still has some members.
Apartment 26 never, as far as I know, had 26 members nor resided/recorded in Apartment 26.
Aphex Twin is just one guy.
Apollo 440 is way off.
Authority Zero has more than zero members (although it shouldn’t).
Destroyer 666 prolly haven’t even played 666 live shows, let alone had 666 members or 666 songs or anything.
Sevendust has never had 7 members.
Seventeen not only didn’t have 17 members, none of the members were 17.
9th Wave is a surf band with fewer than 9 members and is not the 9th iteration of the band.
11 Paranoias is just 3 guys.
16 Horsepower never had 16 members (or even 8).
400 Blows did not reference how many times you’d get punched in the face in one of their mosh pits.
1000 Homo DJs was fewer than 1000 people, none of them (to my knowledge) is gay and none of them are or were DJs.
Andre 3000 is just one guy.
Brasil '65, Brasil '66, Brasil '77, Brasil '86, etc. (all named for years, of course).
Johnny Cash’s backing band The Tennessee Two changed its name to The Tennessee Three when a drummer was added. I don’t know if there was ever a time when the name didn’t fit the number of musicians.
Some bands, on the other hand, strive for consistency. The four letters in Wire represent the members of the band. When one guy left the band, the remaining three performed as Wir until they replaced him with a new member.
Luckily, both “quintet” and “quartet” start with “Q”, or NRBQ would have qualified for this thread.
Three Dog Night has seven members, though
