I list 3 phone numbers under 2 factor authorization, but it still only sends a code to the one main phone number

Two-factor authentication automatically sends codes to all trusted devices, not to trusted phone numbers. If you want a code sent to a trusted phone number you need to request that on a case by case basis and can specify which number at the time the code is requested. See information in the "Get a text or phone call" section of Get a verification code and sign in with two-factor authentication - Apple Support