"Quinn! You spoke to us!" ― Jamie, "Fat Like Me"
Three students at Lawndale High, The Three Js are boys obsessed with one of their classmates, Quinn Morgendorffer. Quinn was only able to remember the names of Joey and Jeffy, often calling Jamie "Jameel", "Jason," and other names that began with "J." Ironically, Jamie is the only character to be given a last name in canon. In the shooting script for "The Invitation", they're described as "three nearly identical jockboys".
By the process of elimination on the ending credits, we know the voice actors for the three are Steven Huppert, John Worth Lynn, Jr and Tim Novikoff. We don't know which was which.
Though in competition with one another for Quinn's affections, and having come to blows in The Invitation, the characters appear to share the same brain, and together are almost smart enough to be considered a single person of only marginally-below-average intelligence. They often appear to think as a collective, such as in Groped by an Angel, when they manage to collectively arrive at the conclusion that Quinn's "guardian angel" is actually a human suitor who is "way too old for her." They also convince themselves, somehow, that Timothy O'Neill, the "Language Arts" teacher, is Quinn's guardian angel.
Quinn uses them without any sign she cares about them or the effort they put into her. Early on, she admitted she didn't want to go steady with any of them but still tells them to keep "paying attention to me and buying me stuff and driving me places". Even though she picks them in "One J at a Time" when she seeks a steady boyfriend, she writes them off for such 'failures' as needing to pee during her lengthy phone calls and being unable to take her to a sold-out concert. In MTV's feature "Love is a Way Too Many Splendored Thing", Quinn sums them up: "These guys are fun to hang out with, except when they are fighting over me, which is even better."
"The Daria Database" took their obsession over the line and showed us their "Quinn Central Reconnaissance Room", where they keep a record of Quinn's routes to school, her class schedule, a black phone to report in Quinn sightings (mainly used by Jamie to send them to false locations), lovingly-preserved items Quinn borrowed (and broke), and a Red Phone set aside solely for Quinn to call. "It has yet to ring."
The Three Js character page on Outpost Daria (via Wayback Machine)