The eddy currents induced in three dimensional structures of in-vessel components in tokamaks and their influence on the plasma controls are investigated. The in-vessel components in ITER are used as an example. A simple method of modifying a toroidally symmetric two dimensional (2-D) model to reproduce the effects of three dimensional (3-D) in-vessel components is proposed. Several issues in plasma control are examined to determine whether the 2-D model adjusted by this method reproduces the results of the 3-D model. Analyses are performed for the plasma stabilization effects of in-vessel components, the evolutions of plasma position and shape in plasma disturbances, the magnetic probe signals and the stray fields at plasma breakdown. It is shown that this method of modifying 2-D models is acceptable for simulating plasma motions and probe signals. However, discrepancies between 3-D and 2-D models are observed in the probe signals during rapid motions of plasmas and stray fields at plasma breakdown. Plasma control issues, for which attention has to be paid to 3-D paths of eddy currents, are discussed