There is nothing mutually exclusive about accepting an in-kind gift that is unrestricted as to its use. It sounds as though you would have an in-kind gift that is unrestricted if you accept title to the three houses and then sell them.

You may, however, want to ask the trustee to sell the houses to your buyer and then distribute the proceeds to you.  Normally, a trustee must consider the best interests of the beneficiary in deciding how to distribute the remainder of the trust and you may be better off getting the cash without accepting title.  Assuming the buyer will pay the trustee the same amount the buyer has offered to you, the trustee may be able to complete the transaction more quickly (transferring the real estate taxes and insurance costs to the buyer more quickly), may avoid a transfer or recording tax that might be imposed on the deeds to you in some jurisdictions, would relieve you of the necessity of obtaining insurance on the properties (even if only for a short time), and would relieve you of any costs you might have to pay professionals to help engineer a second set of transfers.  More importantly, a direct transfer would keep your organization out of the chain of title and less susceptible to litigation if there is something wrong with any of the properties. 

If you don’t intend to use the properties and will only hold them until you can flip them to your purchaser, letting the trustee do all the work may be the smartest way to go.  Then you can record the receipt of unrestricted cash on your financial statements and tax return.