A outside wall mounted 100amp 3meter/breaker-combo from a
gas meter, replacement for single meter serving 3 apartments

Table 110-26(a)
What is the voltage to ground?
Assuming 120V- 3ft deep by 30inch wide or the width of the equipment, which ever is wider.

Hotwire,

Where I am there must be a 12 inch clearance from the Gas. You should ask your local utility what they require.

Bill