Son Heung-min and Harry Kane were both given the freedom of the Wembley pitch to score for Spurs either side of Andriy Yarmolenko's stunning effort for the Germans.
Kane bagged a third for the hosts after the break and Bosz conceded that his side did not do enough to combat a potent Spurs attack.
Dortmund host reigning champions Real Madrid in a fortnight with Dutchman Bosz knowing his side must improve dramatically if they are to get anything out of the game.
He said: "It was clear today that we weren't well organised at the back defensively. I thought we started well, dominated and controlled the game with good football.
"But when you play like that, with space behind the back four, you have to defend it, and we didn't do that well enough.
"We were not there for the decisive moments twice in the first half. We controlled the match, had good possession, but ultimately failed. Twice our defence was not well organised: that must not happen.
"Pressure is always coming from the outside. We played well today, but did not win. We must defend better in the next game. But we played good football today."