In this paper, an 8-element Ultra-wideband Multiple-Input-Multiple-Output (UWB-MIMO) antenna system is proposed. This MIMO system has novel miniaturized antenna elements that are evolved from a conventional monopole patch antenna. These antennas incorporate various impedance matching features including but are not limited to smoothly tapered fed lines, split and truncated ground configuration, semi-circles arches, and inverted p-shaped slots. The antenna pairs are arranged in the cuboidal form to achieve a 3D-arrangement whereas customized decoupling structures achieve the desired isolation levels for three configurations including side-by-side, orthogonal, and across. These three distinct isolation mechanism offers isolation ranges from 20-30 dB for the proposed configurations. More importantly, a diamond eye structure (DES) as a parasitic element is also designed and optimized to improve the impedance matching owing to the lossy nature of isolation structures. The proposed antenna system has compact dimensions of $26.9\times 26\times26.9$ mm3 and its investigated MIMO performance proves its suitability for communication devices operating in the whole UWB spectrum of 3.1-10.6 GHz.