There is now overwhelming data suggesting roughly half of all sun-like stars are in binary or multiple systems and nearly all massive stars form stellar companions. Despite the ubiquity of binary and higher order systems, the formation processes leading to stellar multiplicity remains an outstanding problem in astrophysics. Nonaxisymmetric instabilities have been studied in conjunction with the formation and evolution of astrophysical systems. In protostellar systems, these instabilities provide an avenue for the transference of angular momentum between star and disk and can lead to disk fragmentation and planet formation through development of oscillation modes and resonances in the protostellar gas and dust. The nonlinear evolution of three-dimensional (3D) protostars stars surrounded by protoplanetary disks is computationally modeled to understand the conditions under which short-period binary star systems could form and to probe models of circumbinary Jovian planet formation. To focus this broad study, we consider a system with Md / M_* = 1.18 where Md and M_* are the masses of the disk and star respectively. The star is in differential rotation with b|= T / |W| = 0:22 - the ratio of gravitational potential energy to the absolute value of the rotational kinetic energy - is below the Newtonian threshold value 0.27 where barlike instabilities have been shown to develop for incompressible Maclaurin spheroids. We find that the circumstellar material tends to reduce the inward force of gravity which lowers the threshold in b where dynamice barlike nonaxisymmetric instabilities develop compared to systems without circumstellar disk material. In the system tested, an m = 2 barmode instability develops in the central star. We determine characteristic wave amplitudes and frequencies for the fastest growing modes. Once the instability reaches saturation amplitudes, the central bar interacts with the disk and the system evolves into two relatively equal mass central objects which rotate on independent axes and orbit a common center of mass.