What was the first superhero team in comics?
To avoid broad interpretations, by superheroes I also mean the ones with or without superpowers just like Young Allies, Batman, Bucky Barnes, Rocketeer / etc.
In conventional comic books, this is most likely the:
According to DC comics, the JSA is considered the first "official" super hero team to be formed:
The JSA’s first appearance came in 1940 in the pages of ALL-STAR COMICS #3, making them the very first official super hero team to exist.
DC Comics 101: What's the Difference Between the Justice Society and the Justice League?
First appearing all the way back in Winter of 1940-1941, in All Star Comics #3, the Justice Society of America would become the inspiration for what is currently known as the Justice League of America
Funny you should mention them, because (possibly) the second team in comics was in fact the:
Their first appearance coming just 6 months after the JSA in the Summer of '41, in Young Allies #1, consisting of Bucky Barnes and Toro
...decided to gather a group together to fight juvenile crime. They gathered Percival Aloysius O'Toole, Jefferson Worthington Sandervilt, Henry Tubby Tinkle and Whitewash Jones together and formed the Young Allies.
Young Allies - Marvel.com
Coming just after the Youn Allies, in (possibly) third place is:
Making their first appearance in the winter period of 1941, appearing in Leading Comics #1, a full year after the first team had been formed.
The Seven Soldiers of Victory (also known as the Law's Legionnaires) was a super-hero team, active during the 1940's. Its members were the Vigilante, the Crimson Avenger, Green Arrow and Speedy, the Shining Knight, and the Star-Spangled Kid and Stripesy.
Seven Soldiers of Victory - DC Wikia
Justice Society of America in 1940
Taken from Wikipedia
The Justice Society of America was conceived by editor Sheldon Mayer and writer Gardner Fox. The JSA first appeared in All Star Comics #3 (Winter 1940–1941), making it the first team of superheroes in comic books.