The 15-35 mk3 was announced at the same time as the Canon 5D mk4 camera and 24-105 F4L MK2 lens (August 2016)
This from the press release:
New Canon EF Lenses and EOS Accessories
In addition to the new EOS 5D Mark IV DSLR, Canon is also introducing two EF-Series L-series lenses as well as a variety of EOS accessories. The new EF 16-35mm f/2.8L III USM Ultra-Wide Zoom Lens features a large diameter GMO dual surface aspherical lens and ground aspherical lens, f/2.8 aperture throughout the entire zoom range, fluorine coating, improved durability and is dust and water resistant. The new EF 24-105mm f/4L IS II USM Standard Zoom lens features an improved four-stop image stabilization as well as ghosting and flare reduction with air sphere coating. The Canon EF 16-35mm f/2.8L III USM Ultra-Wide Zoom Lens is scheduled to be available late in October for an estimated retail price of $2,199.00†† and the EF 24-105mm f/4L IS II USM Standard Zoom Lens is scheduled to be available in late October for an estimated retail price of $1,099.00††.
Additional EOS accessories include the Canon Battery Grip BG-E20, Canon Rain Cover ERC-E5S/E5M/E5L and Canon Protecting Cloth PC-E1. For more information about all these products and accessories, please visit: https://www.usa.canon.com/5D4Legend
††Availability, prices and specifications are subject to change without notice. Actual prices are set by individual dealers and may vary.