Acts on mitochondrial thioredoxin 3. Implicated in the defense against oxidative stress. [thioredoxin]-dithiol + NADP(+) = [thioredoxin]-disulfide + H(+) + NADPH Binds 1 FAD per subunit. Homodimer. Present with 414 molecules/cell in log phase SD medium. Belongs to the class-II pyridine nucleotide-disulfide oxidoreductase family.