Is UNIQUE-X3C a US-complete problem?
Yes, it is.

This scanned chapter 3 from Garey-Johnson classic book describe a parsimonious reduction from 3SAT to 3DM. And X3C is a generalization of 3DM. So, the same reduction can be used to parsimoniously reduce 3SAT to 3DM.

Thus, $\mathrm{UNIQUE-3SAT}$ many-one reduces to $\mathrm{UNIQUE-X3C}$. As a consequence, $\mathrm{UNIQUE-X3C}$ is $\mathrm{US}$-complete.