Determining Market/Comperable Rents for Triplex

[Erik R.]

Hello All-

I've come across a potential triplex deal in my area, and have a question as I work through my cash flow assessment spreadsheet. 

The seller of this property disclosed his full profit and loss statement for the past 3 years, and I believe his rents are under market.

What is the best quick and dirty way to assess market rents for the units within a duplex/triplex/quadplex? For SFH evaluation Rentometer.com is a great tool.. can it also be used for this triplex I'm looking at?

thanks

Erik