I have a Dell Inspiron 13 with default settings, original hardware and windows and no virus.
For the keyboard settings I use English (Australia) and Portuguese (Brazil ABNT). With Australia Settings I get open brackets [ and with Portuguese I get closing brackets ] when the 3 letters are pressed at the same time.
Sample of pressing them together 5 times: tit[it[ritr[i
You can try reinstalling the keyboard driver and see if this helps. However, keys are not meant to be pressed together unless it is like alt+an F# key, etc. It isn't uncommon for pressing many together (that aren't meant to be) to give you odd results. It really depends on the keys being pressed and the layout you have it set to use at the time.