Sylvia is a triple asteroid system located in the main belt . We report new adaptive optics observations of this system that extend the baseline of existing astrometric observations to a decade . We present the first fully dynamical 3-body model for this system by fitting to all available astrometric measurements . This model simultaneously fits for individual masses , orbits , and primary oblateness . We find that Sylvia is composed of a dominant central mass surrounded by two satellites orbiting at 706.5 \pm 2.5 km and 1357 \pm 4.0 km , i.e. , about 5 and nearly 10 primary radii . We derive individual masses of 1.484 _ { -0.014 } ^ { +0.016 } \times 10 ^ { 19 } kg for the primary ( corresponding to a density of 1.29 \pm 0.39 g cm ^ { -3 } ) , 7.33 _ { -2.3 } ^ { +4.7 } \times 10 ^ { 14 } kg for the inner satellite , and 9.32 _ { -8.3 } ^ { +20.7 } \times 10 ^ { 14 } kg for the outer satellite . The oblateness of the primary induces substantial precession and the J _ { 2 } value can be constrained to the range of 0.0985 - 0.1 . The orbits of the satellites are relatively circular with eccentricities less than 0.04 . The spin axis of the primary body and the orbital poles of both satellites are all aligned within about two degrees of each other , indicating a nearly coplanar configuration and suggestive of satellite formation in or near the equatorial plane of the primary . We also investigate the past orbital evolution of the system by simulating the effects of a recent passage through 3:1 mean-motion eccentricity-type resonances . In some scenarios this allow us to place constraints on interior structure and past eccentricities .