There is a federal standard of identity for calling something "Salisbury Steak", and it requires that the meat content must be predominantly beef. However, up to 25% of the meat "block" can be pork ("fully certified beef heart meat" is also permitted). No poultry is permitted in the meat block under the standard definition.

But there is a way around this (isn't there always?). Because they are deviating from consumer expectations for traditional all-beef Salisbury steak, the USDA prohibits them from using the unqualified name, "Salisbury Steak".

To avoid having to call it a "Beef, Pork, and Chicken Patty" (ugh!), the USDA allows a modified product name. The phrase "made with chicken, pork and beef" is an explicitly required, legally binding descriptive modifier. It tells the consumer: "This looks like Salisbury steak and is processed similarly, but we changed the meat formulation to include non-standard species." The Banquet product seems to follow that route, though I think a product that has more chicken and pork than beef is really stretching the name "Salisbury Steak".

Incidentally, the standard also allows fillers like cereal, breadcrumbs, cracker meal, or soy protein isolates to be added, but they are strictly capped at 12% of the total product weight (or 6.8% if using isolated soy protein). See, for example, the Lean Cuisine product.