Their lease will end by March 31, 2018.

I think there are few different ways to do this (e.g: lease option, subject to, land contract) Check out this article to get started then search the individual tactics in the BP search bar to get more info. https://www.biggerpockets.com/renewsblog/2011/07/20/seller-financing-3-kinds/

We put a tenant with a lease option.

Is this one lease that all three of them sign, or three leases, one for each of them.If one of the three does not pay, then what?

On the lease it is not specified as a "move-in fee" but only referred to as the non-refundable portion of the deposit.

If you can be fair and balanced with your tenants, yet be stern and stick to your lease agreement, then you will make it. 

Also, learn how to buy properties with seller financing or Subject To, or with leases and options.

It's not the prettiest thing but throws off decent cash flow for the investment.The property is just under 3 acres with 6 leased lots with tenant-owned homes and a 2/1 block building. 

From what the seller has told me all of the tenants are willing to renew their leases, and the commercial unit is still under lease for another 2 years. 

If you find it’s not for you, you don’t like talking with tenants, coordinating lease renewals, etc, by all means - find a GOOD PM.