The phylum with soft bodied animals with a true body cavity and a 3 part body consisting of a foot, visceral mass, and a mantle

Answer is phylum Mollusca.

The members of the phylum Mollusca are soft bodied marine animals but some are found in fresh water or on land. Members are usually bilateral. Few are secondarily asymmetrical (snail) due to twisting (snail) during growth. These are triploblastic with system level have true body coelom. The body is divided into three regions: head, visceral mass and foot. Calcareous shell around the body in most cases. Examples are snail, octopus, sepia, etc.

I'm pretty shure that belongs to the phylum Mollusca