Yes, it is officially the third kit!
From Arsenal.com:
"Arsène Wenger's side will wear this kit for away fixtures in Europe when Arsenal's red and white colours clash with their opponents' kit.
The new strip will be the second-choice change kit for domestic fixtures and will get its first airing when Wenger's team take part in the Amsterdam Tournament on August 2 and 4.
Arsenal's third shirt is made with a state-of-the-art Fit-Dry fabric which wicks away perspiration to help keep you dry and comfortable."
