I skip the threatening part and go right to an attorney letter. 

A copy of the Pre-Approval letter along with an offer goes a lot further than stating that you have been "Pre-Qualified".

Talk to a real tax account (not hr block or turbo tax) and ask them for their help.

Want and ability are two very different things, sort out which this is before you waste a ton of time on details and logistics.A tenant that wants to buy a $300k house for $150k is not worth your time ironing out all the details, once they show you a preapproval letter for what you are willing to sell for then you can figure out the rest.

Any advise on a way around this road block?

You can report it to a collection agency but first send them a certified letter stating that this is a formal request and if they do not comply you will have a debt collection agency go after it and it will effect their credit and lending opportunities in the future - you will have to do it for all 3 because they're responsible for it collectively.

I feel uncomfortable sending a letter with so much personal information . 

Simple solution was to send a letter to all the tenants stating they would be charge if found guilty but they failed to do so.

I started this as I heard a horror story of a tenant providing a letter from a housing assistance organization. 

When you make offers on properties do you always have a pre-approval letter?