(Cont.) This method of differential astrometry is applied to three star systems. Equulei is among the most well-studied nearby binary star systems. Results of its observation have been applied to a wide range of fundamental studies of binary systems and stellar astrophysics. PHASES data are combined with previously published radial velocity data and other previously published differential astrometry measurements to produce a combined model for the system orbit. The distance to the system is determined to within a twentieth of a parsec and the component masses are determined at the level of a percent. n Pegasi is a well-known, nearby triple star system consisting of a "wide" pair with semi-major axis 235 milli-arcseconds, one component of which is a single-line spectroscopic binary (semi-major axis 2.5 milli-arcseconds). Using high-precision differential astrometry and radial velocity observations, the masses for all three components are determined and the relative inclination between the wide and narrow pairs' orbits is found to be 43.8 ± 3.0 degrees, just over the threshold for the three body Kozai resonance. The system distance is determined to a fifth of a parsec, and is consistent with trigonometric parallax measurements. V819 Herculis is a well-studied triple star system consisting of a "wide" pair with 5.5 year period, one component of which is a 2.2-day period eclipsing single-line spectroscopic binary. Differential astrometry measurements from PHASES determine the relative inclination of the short- and long-period orbits. Finally, the prospects for finding planets that simultaneously circle both stars in a binary system are evaluated. Planet searches of this type would represent a complementary investigation to PHASES and contribute similar scientific results