How to excavate 3ft deep around perimeter of building
Who told you that you need insulation 3’ down around the perimeter of the building? Let’s try this a different way:
The Building Code is divided into several sections, including the Energy Code, Structural Code, Fire Code, etc. Your question references the Energy Code.
The Energy Code identifies the minimum energy code requirements. These requirements are outlined in what the Code defines as the “Prescriptive Path”. That is to say, if you use the minimum values of each item in the Prescriptive Path, (i.e.: R-value of insulation in attic and walls, U-value for windows and doors, efficiency of heating units, etc.) then you comply with the Energy Code.
However, you can substitute a higher value for one item in order to reduce (or eliminate) another item.
So, if you don’t have perimeter insulation 3’ down around your slab, you may be able to increase the R-value of your attic insulation or purchase a more energy efficient heating unit, etc.
In order to change the items in the Prescriptive Path you’ll need to hire an architect or mechanical engineer, but they could save you $$$$ and a ton of grief.
Btw, I noticed that you’re converting a building into “residential units”. If the building will have more than 2 units, you’ll need fire sprinklers, unless you install a 2-hour fire wall between each 2 units. (An architect can outline how to do that.)