Buying properties that are not for sale

All,

This is my first post, so any constructive criticism is welcome.

I am looking to buy three properties in the NY/LI area near where I live. 

1) 23,278 sq. ft. - Not for sale

2) 27,520 sq. ft. - Not for sale

3) 6875 sq. ft. - For sale/Pre-forclosure

I would like to take these three adjacent properties and combine them all to build 6-8 houses or rental complex (attached housing) including a new street/cul-de-sac. Working in the real estate development field for 8 years and overseeing the development and design of large (30-40 story) hotels, office buildings etc, I am not newbie. I plan on going on my own eventually and this might be my first go. Being knowledgeable in the financial part has helped but since I don't practice it, therefore it is just knowledge and not experience. My real question is how would someone approach a specific owner who doesn't have his property up for sale? Through a broker? Attorney? Knock on their door? Mailing?

Any and all advice would be great!

Thank you,

Yossi