In your Steam Library, change the category to Tools and Arma 3 Tools will be in the list; the suite is about a 213 MB download. Note that "Launch Game" will simply result in Addon Builder (the Arma 3 version of the prior BinPBO Personal Edition tool for packing mods), while right-clicking it will instead give the option to use Oxygen 2, the Arma modelling tool; you'll need to go into the mod tool suite folder (i.e. C:\Program Files (x86)\Steam\steamapps\common\Arma 3 Tools) in order to run other included tools.

Also note that if you right-click Arma 3 Tools in your Steam "Tools" Library, go into its Properties, click the Betas tab, and choose the "licensed_data_packages - Huge Data Packages" option, it'll download 22 GB of licensed source data from the prior games usable for making licensed Arma 3 mods with; you can find them separately here instead.