The Little Mermaid comes out in the on Blu-Ray October 1st in the US. The sequel and prequel Blu-Ray combo pack, The Little Mermaid: Ariel's Beginning / The Little Mermaid: Return to the Sea will be sold on November 19, separately from the first film. A trilogy set has not been announced for the US.
You could buy the movies separately and wait a few months, or just import the UK boxed set right now. The Little Mermaid Collection contains all three films on separate discs in their own keep cases. The set is confirmed region-free. I believe the discs are identical to what we'll get in the US (correct me if I'm wrong).
Amazon UK sells The Little Mermaid Collection for GBP 21.58 ($35.92 at present) with standard international shipping. The list price is slightly higher, but that's because of the VAT tax that gets removed when shipping to the US. Pretty nice deal if you're not interested in the 3D version of the first film included in the US Diamond Edition release.