Assembling Lots for Multi-Unit Dev - Negotiating Pitfalls w Owner

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I'm working on piecing together 3 lots with 3 different owners in a hot neighborhood in Denver. Each lot has an existing SFR on it. Zoning is higher density, so the more dirt I get, the more units I can build. I've got the largest lot locked down and I can put 4 units on it, so I dont *need* the additional lots, but having them included will make this project a lot more attractive.

The owner of the adjacent property to the one I have under contract expressed an interest in selling because they are moving next month and had planned to keep their home as a rental. Their asking price is reasonable.

I have not spoken with the 3rd owner yet, who's property sits adjacent to the first neighbor. I've been told his potential asking price, and it is $75k more than I am willing to pay.

My question is this: what are some things I need to consider when negotiating with multiple owners on the same block? If they start speaking with each other it could potentially blow up the deals. Do I include an NDA in the purchase agreement? What are some approaches or tactics that I can use to help navigate this situation more smoothly? 

To be clear, I don't need the other two lots to make this development project work and I have let the first neighbor know this.