I am in the Springfield, Oregon rental market.....shooting-from-the-hip analysis: $500K owner-financed triplex sounds like it is likely a good deal. Perhaps, try to figure out why the seller is stuck on $80k for a down, what is he trying to accomplish, then figure out an alternative way for him to get what he wants.
Is the property in Eugene or Springfield? This is important, Eugene just enacted some restrictive regulations that will increase the cost of operating rental property and risk. Also, Oregon state rent control rules limit to less than 10% per year for rent increases so the under-market unit may end up being under-market for a few years due to legal restrictions.
Why are two of the three units vacant? Are they ready to rent, or will it take your time and money to get them ready?
Area of town can make a big difference on the type of residents that the property will attract and the level of work and risk involved in operating it over time.
Sounds like it is an older property, which is okay, but increasingly lead paint, asbestos, etc. are concerns to residents and government regulators. This translates into increased costs and risks for the owner-manager.
That said, keep checking it out and underwriting the deal, it sounds like there is potential there. If you want to have a cup of coffee sometime and discuss, let me know.