We demonstrate an electromagnetic (EM) cell that can generate uniform electric field with three dimensional (3D) polarization dynamic control for the EM compatibility/interference (EMC/EMI) testing. 3D orthogonal slotline structures are designed and utilized to build the walls of the EM cell. By feeding the slotlines in phase and terminating them with matching loads, the uniform electric field similar to a standing wave is produced by two slotline modes propagating in opposite directions inside the cell. Microwave switches are integrated into the slotlines so that we can choose the operating slotline dynamically and change its orientation in a 3D manner by adjusting the ON/OFF state of the appropriate switches. As a result, the polarization of the EM field generated by the operating slotline will be changed orthogonally in 3D. Both simulation and experiment show that a polarization controllable uniform electric field can be achieved in the center region of the EM cell from DC frequency to its first EM resonance frequency, enabling EMC/EMI testing more flexibly without rotating the device under test.