We report the discovery of one newly confirmed planet ($P=66.06$ days, $R_{\rm{P}}=2.68±0.17R_⊕$) and mass determinations of two previously validated Kepler planets, Kepler-289 b ($P=34.55$ days, $R_{\rm{P}}=2.15±0.10R_⊕$) and Kepler-289-c ($P=125.85$ days, $R_{\rm{P}}=11.59±0.10R_⊕$), through their transit timing variations (TTVs). We also exclude the possibility that these three planets reside in a $1:2:4$ Laplace resonance. The outer planet has very deep ($∼1.3%$), high signal-to-noise transits, which puts extremely tight constraints on its host star's stellar properties via Kepler's Third Law. The star PH3 is a young ($∼1$ Gyr as determined by isochrones and gyrochronology), Sun-like star with $M_*=1.08±0.02M_⊙$, $R_*=1.00±0.02R_⊙$, and $T_{\rm{eff}}=5990±38$ K. The middle planet's large TTV amplitude ($∼5$ hours) resulted either in non-detections or inaccurate detections in previous searches. A strong chopping signal, a shorter period sinusoid in the TTVs, allows us to break the mass-eccentricity degeneracy and uniquely determine the masses of the inner, middle, and outer planets to be $M=7.3±6.8M_⊕$, $4.0±0.9M_⊕$, and $M=132±17M_⊕$, which we designate PH3 b, c, and d, respectively. Furthermore, the middle planet, PH3 c, has a relatively low density, $ρ=1.2±0.3$ g/cm$^3$ for a planet of its mass, requiring a substantial H/He atmosphere of $2.1^{+0.8}_{-0.3}%$ by mass, and joins a growing population of low-mass, low-density planets.