This article proposes a class of ultra-wideband 3-D microwave absorbers with composite slotlines and microstrip lines from synthetic design to practical implementation. Firstly, the general equivalent transmission line (TL) model of the proposed 3-D absorber element is formed based on two sets of shunt short-ended stubs and an i -section nonuniform connecting line. Then, the synthetic procedure is theoretically investigated using the equivalent TL model, aiming to construct an ultra-wide absorption band with the Chebyshev equal-ripple response. According to the prescribed absorptive performance, such as maximum reflection coefficient ( \Gamma ) and fractional bandwidth (FBW), the relevant parameters of the TL model can be directly calculated by the established synthesis approach. To validate the proposed concept, two prototypes, namely, absorber-I ( i\,\,= 1) and absorber-II ( i\,\,= 2), are designed, fabricated, and measured. Each prototype element applies a simple 3-D structure composed of slotlines, microstrip lines, and only one absorptive load, which are all etched on a single-layered substrate. Measured results agree well with synthetizations and simulations. For absorber-I, the measured bandwidth of 95.5% in a frequency range from 5.08 to 14.44GHz is successfully realized. Absorber-II obtains a measured FBW of 111.1% in a range from 4.48 to 15.68GHz. Average absorption ratios within the operating band are higher than 95.5% and 93.8% for absorber-I and absorber-II, respectively. Besides, the proposed 3-D absorbers are angularly stable within 60° oblique incidence. Therefore, such ultra-wideband 3-D microwave absorbers with simple structures possess the attractive potential for effectively absorbing electromagnetic waves.