Luckily the fire department was able to come in time to salvage the home.

To the OP, what I've seen is that people who want to make serious money in real estate, get a degree from a place like Wharton Real Estate Department.

I've found that it is zoned RT1 per this: https://www.stpaul.gov/departments/safety-inspecti...However, it doesn't seem to mention a triplex, especially if it's in a home in a residential neighborhood and not a small building.

Call on your neighbors if they aren't complying,, report their trash, their crap cars, their grass.. the overall curb appeal in the neighborhood will improve as well as the class of tenants.. help raise the bar.. and the rents..Photos of what's going on is fair way to approach the inspection department if you felt your being pressured.. but get in shape before you respond..

We ended up filing a letter of vacancy with the Los Angeles County Sheriff's Department which allowed them to forcibly remove anyone that we considered to be trespassing on our property. 

Even for the most basic of zoning, T3/single family, the lot size minimum always says 5000 sq ft.I've tried emailing and calling the zoning department numerous times and they have never gotten back to me.

This one would fall into the 'you never know' department.

You can start with the delinquent property taxes and also if you need info with the records you can go to the recording, the assessment or appraisal department, maps or gis info, etc. 

I would suggest going to the planning and zoning department, and pulling the building permits to see who is building and where.

But again most folks that are rental geared simply are not interested in this facet ... as it tends to be quite capital intensive and not really all that great in the leverage department.. cash buyers well they have a leg up ..