The crosstalk among three identical uniform coupled microstrip transmissiom lines is examined. To analyze the crosstalk, the simultaneous equations for the voltage and current waves on each transmission line are induced from the transmission line equation. The capacitance and the inductance of the line to solve the transmission line equation are calculated by the spectral domain analysis and the space domian analysis. There are three quasi-TEM modes is three microstrip transmission lines and the characteristics mode impedences in each mode are almost equal at a weak coupling state. The crosstalk among three identical microstrip transmission lines is calculated varying the frequency from 50MHz to 3GHz.