Isodimorphism and isomorphism are becoming mature approaches but separately used to regulate crystallization behavior of random copolymers and thus their performances. Most of the research work is focused on binary copolymers while leaves random copolymers containing more than two kinds of comonomeric units rarely studied in which both isodimorphism and isomorphism may occur together. This experiment is realized in the terpolymers of poly(butylene adipate-ran-butylene succinate-ran-butylene fumarate) (PBASF) wherein butylene adipate (BA) displays isodimorphic behavior with either butylene succinate (BS) or butylene fumarate (BF), and BF builds an isomorphic structure with BS. When the BA content is kept almost unchanged at similar to 50 mol %, PBASF can be subtly tailored from a pseudoeutectic state to only isomorphic state by increasing the BF content that suppresses the crystallizability of the PBA-rich phase but hardly affects the PBSF-rich phase. Crystallization behaviors of PBASF from both melt and quenched samples are assayed in detail by using a differential scanning calorimeter, X-ray diffractometer/scatterometer, and Fourier transformation infrared spectrometer, and the mechanism of confined crystallization is clearly unraveled, which is relied on both intersegment hydrogen-bonding interaction and physical restriction. Furthermore, a crystal phase diagram for random terpolymer of PBASF is proposed, which provides an overall guide to tailor its crystallization. This example of the strategy combining isodimorphism and isomorphism raised here sheds light on precise regulation of properties and performances of terpolymer and copolymer with more complicated chain structures.