In Drupal Commerce, how can I decrease stock of product B when buying product A with rules? I'm having some issues writing some rules for my site.

Let's say I have two products. B and C, as well as a combined product called A. Buying product A should decrease the stock of B and C by 1, but the stock of product A should also always be the lowest stock of B or C.

I can't, for the life of me, find out how to do this, and googling has turned up zero results.
Are you using the Entity Reference module?  You will somehow need to set up the relationships between A + B + C in Drupal, and Entity Reference is one Rules-module compatible way to do this.

Another more flexible but more complicated way would be to use the Relation module.