Currently, my friend and I split the monthly mortgage payment but he pays all utilities.

My business partner and I have been successfully utilizing wholesaling as an exit strategy for the past three years.

Utilize the most efficient form of entities, Series LLC instead of individual LLCs, LLC instead of corporation (you can elect to have LLC TAXED as a corporation if that is desirable), LLC instead of limited partnership (eliminates the need for a separate entity to be created as general partner).  

The decision as to which structure to utilize usually depends heavily on whether or not you are eligible for a Solo 401k.

I split my house up into three units. One unit has nearly the full back yard and has a lot of water irrigation. The front yard is a common area for all three units and also has a lot of irrigation. How do I go about d...

vacancy, repairs, capex, management, any shared utilities, any groundskeeping not done by the tenants, any permits or registration fees, etc. 

Hey Elijah, Utilizing MLS access for comps is the most reliable method, but if it's not available, you can still utilize the parameters Jonathan mentioned on other platforms like Zillow, Propstream, and Batchleads.

While the bookkeeping will likely be complicated, is the accounting/taxation on this STR based on 100% STR income/expenses allocated 100% to business and general house expenses (mortgage, utilities, depreciation etc) pro rated between STR/personal residence based on sq footage?  

However, if have several rentals and need to deposit checks, or use a debit card to pay utilities and other bills, etc. conventional business bank account would be the best choice. 

If the Multi-Fam does not have individual utility-meters for all units, then you have to figure out what to do or it can potentially eat away at your cash flow.  99% of Single Family homes do not have this problem.These are just a few pros and cons.