The pork loin and chicken can be smoked at the same time, but I would do the salmon separately. Salmon has a strong distinct flavor, and if you put it on the top rack, the drippings may alter the taste of the pork and/or chicken. If you place it on the bottom rack, it may no longer taste like salmon. If you are smoking chicken parts, or butterfly chickens; the chicken and the loin should be done around the same time.
When cooking meat and poultry together, many like to place the chicken on the bottom, to prevent cross contamination. I have friends that don't like pork drippings on their chicken, though they will tolerate the smoke from the drippings that hit the drip tray. So I generally will place the chicken above the pork. The safety issue with cross contamination is when the meats are raw.