How do you change a 3 phase air handler into a single phase air handler without replacing the air handler?

You call a very experanced electricial,

Research the situation and fix it yourself ! Change the blower motor to a single phase motor and only use single phase breakers and writing . Or only one of the 3 breakers , Quite simple ! I know I am stupid but what do you expect with a name like bikertrash123 ?

NOT simple. The motor is the easy part since many of the motors used on 3 phase are actually single phase to start with so you can probably still use the motor over but the heaters will have to be sounded out with a Amprobe to prevent over current leg conditions.

Besides all of this good stuff when you do the conversion you will destroy the UL/CSA listing and labeling which means the unit will not be safe to operate since the heaters were designed to run at a lower amp draw do to the fact that they are 3 phase. All the safety devices/over curent devices/limit controls will be out of wack for the safe operation of the unit. This is why it was recommended for you to get a real good electrician so it can be done as safe as possible. Hopefully the electrician will also replace the safety devices with ones that coinside with the conversion.