We have analyzed XMM-Newton, Chandra, and Suzaku observations of three young Type Ia supernova remnants (SNRs), i.e., Kepler's SNR, Tycho's SNR, and SNR 0509-67.5 in the LMC, to investigate the properties of both the SN ejecta and the circumstellar medium (CSM). By simply comparing the X-ray spectra, we find that line intensity ratios of iron-group elements (IGE) to intermediate-mass elements (IME) for Kepler's SNR and SNR 0509-67.5 are much higher than those for Tycho's SNR. We therefore argue that Kepler is the product of an overluminous Type Ia SN. This inference is supported by our spectral modeling, which reveals the IGE and IME masses respectively to be 0.95 (0.58-1.29) M and 0.12 (0.07-0.31) M (Kepler's SNR), 0.75 (0.6-1.26) M and 0.34 (0.09-0.42) M (SNR 0509-67.5), and 0.35 (0.2-0.9) M and 0.70 (0.42-0.82) M (Tycho's SNR). On the other hand, there are a number of dense, N-rich CSM knots in Kepler's SNR, which were found by optical observations and are now confirmed by our X-ray observations. Their optical proper motions as well as X-ray measured ionization states indicate that they were located a few pc away from the progenitor system at the SN explosion. Therefore, we argue that Kepler's SN was an overluminous event that started to interact with massive CSM a few hundred years after the explosion, which supports the possible link between overluminous SNe and the so-called “Ia-CSM” SNe.