House Hacking a Multi-Family Unit - tax question

[Luke Spence]

Hi everyone - 

I'm looking at purchasing a 3 unit and living in one while renting out the other two.  I understand that I can divide the expenses and those that are related to the rental property can be tax deductible while those that are related to my personal dwelling are not.  The particular unit that I would be living in is about 1200 sq ft while the other units are both much smaller.  My question is, do I divide the property 3 ways because there are 3 units (so 2/3 of the property is classified as a rental) or do I make the splits based on square footage?  Is it my choice?  It's in California if that makes a difference.  

Thank you!