Composite Galactic supernova remnants (SNRs) are a morphological class of sources that exhibit a shell-like radio morphology coupled with a complex X-ray morphology. This X-ray morphology includes a prominent pulsar wind nebula coupled with diffuse thermal emission that is center-filled rather than shell-like, which is typical of Galactic SNRs. As a distinct class of sources, composite SNRs remain poorly studied in comparison to other classes of SNRs and many of their general properties (in particular, their evolution with time) have yet to be studied in systematic detail. We have undertaken an investigation of composite SNRs using archival X-ray observations made by such observatories as Chandra, Suzaku and XMM-Newton: in this talk I present the results of a spatially-resolved spectroscopic analysis of the composite Galactic SNR 3C 396 (G39.2-0.3) using data from pointed observations made by Chandra and Suzaku. Our results include the detection of an ejecta-rich interior X-ray plasma that features substantial amounts of calcium, a unique discovery amongst Galactic SNRs.