I just did a bit of checking, and unless I've seriously overlooked something, that 1987-08-03 show is indeed the last instance of more than two b-sides in a single gig. Later on the JT Tour, they certainly came close to doing it again but didn't. On Elevation, it could have happened: Party Girl and Spanish Eyes were played in Barcelona on 2001-08-08 and Sweetest Thing was still in rotation at that point, but didn't appear at that show. And in any case, by that time, Sweetest Thing had been re-released as a single.
I don't know of any instance of more than three b-sides in a show. I do not believe it has happened if you strictly define b-side. However, if we take a period of time where a song was a b-side and did not make an album until later, such as mid-1980 before Boy's release when Stories For Boys and Twilight were still b-sides, then some shows may have had more than three b-sides. Unfortunately, we have only
one full set from the period of time between 11 O'clock Tick Tock's release (which took U2's total of current b-sides to four: SFB, Boy/Girl, Twilight, Touch) and Boy's release, and it has just one then-current b-side, Stories For Boys; Things To Make And Do was then an unreleased track.
By no means am I willing to discount the possibility that at some point in 1980, U2 played a set that featured four or more songs that were then currently b-sides, but right now, the best we can do is three in a show.