We investigate the process of formation and subsequent evolution of prominence plasma in a filament channel and its overlying arcade. We construct a three-dimensional time-dependent model of an intermediate quiescent prominence. We combine the magnetic field structure with one-dimensional independent simulations of many flux tubes, of a three-dimensional sheared double arcade, in which the thermal nonequilibrium process governs the plasma evolution. We have found that the condensations in the corona can be divided into two populations: threads and blobs. Threads are massive condensations that linger in the field line dips. Blobs are ubiquitous small condensations that are produced throughout the filament and overlying arcade magnetic structure, and rapidly fall to the chromosphere. The threads are the principal contributors to the total mass. The total prominence mass is in agreement with observations, assuming a reasonable filling factor. The motion of the threads is basically horizontal, while blobs move in all directions along the field. The peak velocities for both populations are comparable. We have generated synthetic images of the whole structure in an H$alpha$ proxy and in two EUV channels of the AIA instrument aboard SDO, thus showing the plasma at cool, warm, and hot temperatures. The predicted differential emission measure of our system agrees very well with observations. We conclude that the sheared-arcade magnetic structure and plasma behavior driven by thermal nonequilibrium fit well the abundant observational evidence for typical intermediate prominences.
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