Gervinho is suffering with a mild muscular problem and Koscielny has damaged his ankle, while Walcott limped off during Saturday's victory over Bolton with what was later described as a minor knee injury.
All three players remain in contention for Sunday's derby clash with Tottenham at White Hart Lane, but they have been ruled out of Wednesday's game at the Emirates.
The news comes as a blow to Arsenal at a time when they are already without Jack Wilshere, Yossi Benayoun and Abou Diaby.
While, injuries to Thomas Vermaelen, Johan Djourou and Sebastien Squillaci mean that Alex Song looks likely to fill in at centre-back alongside Per Mertesacker.
The recent news that Wilshere will miss the rest of the year due to surgery is a particularly big blow, but Arsene Wenger was at least pleased that the midfielder's ankle operation went well.
Arsenal and Injuries will never fall apart.