This article presents designs of 3-D microwave absorbers with synthesizable absorptive performance. The synthetic procedure is investigated based on the unified equivalent transmission line (TL) model, which theoretically supports different performances. According to the desired absorptive performance, including the normalized absorptive resistance (NAR) and fractional bandwidth (FBW), the impedance parameters in the equivalent model can be easily obtained from the closed-form calculation. For demonstration, two prototypes are structurally developed and their different absorptive performances are measured. Each prototype element is designed with a single-layered 3-D structure consisting of composite TLs and only one loaded resistor. The measurements are in close agreement with the synthesized and simulated results, demonstrating an FBW of 99.0% (from 4.95 to 14.66 GHz) for absorber-A with a smaller NAR of 0.53. Absorber-B with a larger NAR of 1.25 obtains a measured FBW of 84.3% (from 5.76 to 14.16 GHz). These two absorbers realize in-band absorption ratios of 92.4% and 93.0% on average. In addition, efficient absorption can be obtained within 45° oblique incidence. Therefore, such a type of the proposed 3-D microwave absorber with synthesizable absorptive performance is indeed a promising candidate to customize electromagnetic wave absorption.