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We present new Atacama Large Millimeter/submillimeter Array observations toward a compact (∼230 au separation) triple protostar system, L1448 IRS3B, at 879 µm with 0.''11 x 0.''05 resolution. Spiral arm structure within the circum-multiple disk is well resolved in dust continuum toward IRS3B, and we detect the known wide (∼2300 au) companion, IRS3A, also resolving possible spiral substructure. Using dense gas tracers, C¹⁷O (J = 3 →2), H¹³CO⁺ (J = 4 →3), and H¹³CN (J = 4 →3), we resolve the Keplerian rotation for both the circum-triple disk in IRS3B and the disk around IRS3A. Furthermore, we use the molecular line kinematic data and radiative transfer modeling of the molecular line emission to confirm that the disks are in Keplerian rotation with fitted masses of 1.19_–0.07^+0.13 M_☉ for IRS3B-ab and 1.51_–0.07^+0.06M_☉ for IRS3A and place an upper limit on the central protostar mass for the tertiary IRS3B-c of 0.2 M_☉. We measure the mass of the fragmenting disk of IRS3B to be ∼0.29 M_☉ from the dust continuum emission of the circum-multiple disk and estimate the mass of the clump surrounding IRS3B-c to be 0.07 M_☉. We also find that the disk around IRS3A has a mass of ∼0.04 M_☉. By analyzing the Toomre Q parameter, we find the IRS3A circumstellar disk is gravitationally stable (Q > 5), while the IRS3B disk is consistent with a gravitationally unstable disk (Q < 1) between the radii ∼200-500 au. This coincides with the location of the spiral arms and the tertiary companion IRS3B-c, supporting the hypothesis that IRS3B-c was formed in situ via fragmentation of a gravitationally unstable disk.