Don't use the metric system. A 12 inch sub can easily be divided into three four inch sections. A yard long sub can be sliced into three foot longs. A 100 cm sub, on the other hand, by definition cannot because the result is irrational.
Flip a three-sided coin to decide who gets the whole sandwich. It will be envy-free before the coin flip (but not after). In general, eg for utility functions with diminishing returns, this solution will not yield the highest utility.
All depends on the trimmings, and what kind of sandwich. Seriously.