What's your exhaust setup on top? It's also possible you don't have enough airflow draw.
Also, when you ssay all three intakes open, do you mean do you have three 3/4" intakes plus a ball valve 4th intake? Or only 3 intakes total?
~ Scott ~
Blue Weber Performer + Craycort CI • Brick Red Weber One-Touch Platinum • Weber 26.75 Kettle • Akorn Kamado • ET 732 • Record-crushing ORANGE Thermapen • UDS In Progress