  The direct summand conjecture asserts that if R is a regular local ring and S
is a module-finite R-algebra containing R, then R is a direct summand of S as
an R-module. It was previously known to be true if R contains a field or if dim
R is at most two. In this article, the result is demonstrated for mixed
characteristic rings of dimension three. The proof is accomplished by showing
that an extension of plus closure has the colon-capturing property in dimension
three.
