This is an increasingly frequent question, so I'll lay it out in detail below.
Here is how it works when buying a game on Steam:
The sale defaults to whatever platform you've purchased on. Then a calculation occurs that tracks which platform you play most in the first 7 days after that purchase. If more play time occurs on a different platform, then the sale shifts over to the most played platform. Its a really smart/fair system. Note: mobile purchases always default to Windows if the game is not played in the first 7 days.
So to recap if you want to support Mac and Linux with your purchase on Steam:
1. Buy the game through your Mac or Linux Steam client.
2. Dont play it on any other platform in the first 7 days after your purchase.
3. If you bought it on your phone, play it on Mac or Linux within 7 days of purchase.
Or you can simply buy it on GameAgent, and don't worry about any of the above since all GameAgent sales benefit Aspyr directly
This is also the case for ANY other retailer. Example: if you buy the Windows version from say Amazon, you are supporting Windows regardless of whether you actually launch the game on that platform. Steam (and Humble Bundle) are currently the ONLY outlets that have platform detection.