Covalent organic frameworks (COFs) are an emerging class of two-dimensional (2D) or three-dimensional (3D) porous structures. Most COFs are typically prepared via solvothermal methods involving long reaction times (> 12 h) that generally result in unprocessable powders, significantly hindering their straightforward integration into devices and application. Current processing methods for 2D COFs have not been implemented for 3D COFs, probably due to the lack of pre-assembly mechanisms among the 3D COF precursors during synthesis. Here, we report a pioneering synthetic environment that achieves two objectives: precise control over the reaction time of 3D COF precursors through controlled diffusion, ensuring the absence of turbulent mixing, and the ability to fine-tune the specific reaction zone where the reaction and controlled diffusion of the 3D COF precursors will occur. We further demonstrate that harnessing these two features in 3D creates an unparalleled synthetic environment, allowing for a rapid one-pot synthesis of 3D COFs, with their formation occurring in less than a minute. Additionally, this approach yields self-standing 3D COF fibers on a centimeter scale, showcasing the versatility and potential of the approach presented for scalable production of 3D COFs as well as for their seamless integration into devices and patterning strategies.