Lots of talk about individuals' deeds and personnel choice. But it's not like a single player cost us the match. For instance, Alba was certainly among the worst, but also probably Barca's most dangerous player. Has to push forward, attack, and leave space behind (which Lenglet didn't really bother covering, still...) I agree with folks saying 433, play with wingers, give or take, would've provided more cover. 3 at the back is not inherently weak or anything, though as everything suits better some situations.
But imo the main problem was the approach. Koeman sent the team to attack like there was no tomorrow since the start. All the flaws mentioned here were emphasized and made more prominent by that. Real won the game early, but feels like Koeman was naive in his approach. Bit like Children's crusade into the Holy Land. They had us where they wanted us.