This is just keyboard ghosting again. Not all keyboards can reliably detect three simultaneous key presses (in this case move left, jump, and shoot). It comes down to the circuitry in your keyboard and so will affect all software on your system.
Closing as not a bug, and please always follow the bug report guidelines in future, e.g. test every browser even if you don't use them normally, and specify the Construct 2 version (we currently provide 2 versions, the latest stable and latest beta).