Justifying low offer on three unit multifamily

[Chris Purcell]

I'm looking to buy a 3 unit multifamily property in Baltimore. The problem is the seller, in my opinion, is asking way too much for it. I want to put an offer in but I don't want to scare them off by offending them or not taking me seriously. 

So of course I need to justify my offer. I need to prove to the seller that no one is going to make any money paying what they want. My question is how do I do this? How detailed of an income and expense estimate should I provide? What kind of explanation should I include with my offer?