Electrodynamic field simulations in the frequency domain typically require the solution of large linear systems. During the design process it is desirable to vary specified parameters like the frequency, geometry details as well as material parameters. Initializing an individual calculation to consider all parameter changes is not efficient. Model Order Reduction (MOR) techniques offer an approach to approximate the transfer function of these multivariate problems in reasonable time. Existing schemes for multivariate MOR require a linearization of the underlying system for systems based on the Finite Integration Technique (FIT). This paper focuses on the linearization for FIT-discretized systems, that depend on frequency and threedimensional rectilinear geometry variations and briefly discusses the utilized MOR techniques.