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."
