Hi Peter and welcome from DFW Texas area!
Just my own small experience: I have smoked chicken on the bottom rack, small brisket on the middle rack and two pork butts on the top rack, all spaced accordingly on my model 3D without any problem. I pulled the chickens first, then the small brisket, about 8 pounds worth and finally the pork butts after about 15 hours total time. I'd put them in at the same time myself, you will smoke them first then they just cook after about 1.5-2 hours. Also, poultry will absorb smoke quicker than the others and finish before the beef/pork will according to its size anyway.
Just my two cents worth, I had a successful smoke that way and would do it again if I needed to smoke more than one type of meat anyway. Smoke all the meat first, I put the meat probe in the chicken breast, pulled the chicken after about 3-4 hours at 165, put the probe in the small brisket to 190 and pulled it and finally, put the probe into one of the pork butts to finish things off at 195 there. A long smoke for three types of meat but they all turned out great!
Carl