  We analyze the influence of noise on magnetic properties of a su-
perconducting loop which contains three Josephson junctions. This circuit is a
classical analog of a persistent current (flux) qubit. A loop supercurrent
induced by external magnetic field in the presence of thermal fluctuations is
calculated. In order to get connection with experiment we calculate the
impedance of the low-frequency tank cir- cuit which is inductively coupled with
a loop of interest. We compare obtained results with the results in quantum
mode - when the three junction loop exhibits quantum tunneling of the magnetic
flux. We demonstrate that the tank-loop impedance in the classical and quan-
tum modes have different temperature dependence and can be easily distinguished
experimentally.
