A 3 foot sandwich can feed between 10 to 15 people depending on how much of the sandwich each person plans to eat.
Most delis will cut a 3 foot sandwich into 2 inch portions which would be a decent serving for many people.
2 foot sandwiches feed around 8 people and 3 foot sandwiches feed around 15 people.
A 6 foot sandwich would feed around 21 to 24 people.
When cutting a 3 foot sandwich you should cut the 3 foot sandwich is 3 to 4 inch slices per servings which will often yield 9 to 12 servings.
However you can cut them a bit smaller if you only want to serve smaller portions.
But it's better to cut at least 2 inches minimum but 3 inches to 4 inches in better.
If you want larger portions of the sandwich served you should cut 4 inches and smaller portions aim for 3 inches.
The thickness of the sandwich can also influence the cut size.
If you don't think you'll have enough then you may want to order a larger sandwich or another 3 foot sandwich to ensure you have enough.