With your 3 yr old it sounds a little more complicated than the standard tantrums. I commend you for taking in a foster child and it sounds like he has some past issues relating to his behavior. Is he okay around your 1yr old? Speak to the agency where you get the play therapy and see if they have suggestions. I bet his behavior probably sets off your other childrens moods too. His situation sounds more challenging and needs more professional advice.