Making a stamina/magicka/health poison with two ingredients produces satisfactory (maximal) results. If, however, third one is added, the poison will have the same strength, same stats, but will use the 2nd stack in your inventory.
Maybe that was intended, although I have hoped for better strength or few seconds longer duration. But still these poisons should stack together.
Bonus: When combining 3 ingredients while making a stamina poison, a full stop mark at the end of the sentence will go missing. xD