Looking at a 3-flat in a mature but strong residential market in Chicago. Rents aren't as high as adjacent neighborhoods, but that's what makes it feasible. I work in real estate, but am used to looking at larger, institutional industrial deals.

What are the most important metrics to think about when evaluating a 3-flat deal? What are typical expense ratios?

Welcome to BP @Ryan Chambers

Well for me the most important is ROI on my cash invested. Without knowing more I estimate at least 50% expenses not including Principal and interest payments.

I agree with Ned. There are tools in FilePlace under the Resources tab to estimate ROI, in case you're looking for one.

In Chicago a big factor for expenses is heat. There's a significant difference between a 3 flat with one shared boiler (which is pretty common) vs a 3 flat with three separated heating systems paid by your tenants. 

I agree with Joey,  the heat cost will vary greatly depending on the system and way it is metered. Otherwise look at the 2015 proposed tax assessment on the cook county site.  They went crazy this year with valuations and I have seen some double, so make sure you are accounting for the possible increase.  .