The problem was that the LCM-3 was designed to carry a 30-ton tank-- such as the M3 Lee. The first models of the Sherman weighed about 33 tons, while later versions weighed in at 42+ tons.
It seems like almost every time a company comes out with a new LCM kit, they release the LCM-3. Trumpeter did it, in 1/35, 1/72, and 1/144 scale. HobbyBoss did it in 1/48. Speedline Models did an RC version in 1/16. Only Dean's Marine led with the LCM-6, in 1/24 scale. (Speedline did respond to customer feedback and now offers an LCM-6 as well.)
The LCM-6 was basically a -3 with an additional six feet of length added amidships. This increased the displacement enought to allow them to carry the Sherman.
The LCM-6 is a much more interesting vessel-- you can build it in WW2 fit, and it was capable of carrying the Shermans. It was also used well into the post war period, into Vietnam. There are all sorts of conversion options for an LCM-6, such as Riverine gunboats or boats with a helicopter landing pad for the UH-1.
Shortly after it first came out, I converted the Trumpeter kit into an RC model. I originally outfitted it with a diecast Sherman tank, but it was really too heavy. I am now running the model with a Dodge Weapons Carrier WC-56, but I am planning on replacing that with an M3A1 Stuart at some point.....