The systematic study of the magnetization, magnetoresistance and magnetostriction of (Th1-xYx)(3)Co single crystals has been performed. At x < 0.3 the magnetic moments of Th ions form a noncollinear magnetic structure with the strong ferromagnetic (F) component along the c axis and antiferromagnetic (AF) components along a and b axes. The magnetic field applied along a and b axes induces the first order magnetization process (FOMP) which is accompanied by a giant magnetoresistance effect (\ Delta rho/rho \ -35%) and magnetostriction (up to 10(-3)) which are strongly anisotropic. FOMP in (Tb1-xYx)(3)Co can be associated with switching of the orientation of local directions of Tb magnetic moments. Both the magnetoresistance and magnetostriction show a significant hysteresis and irreversibility. A large difference in the electrical resistivity in phases with the different magnetic structure and an additional maximum observed in the intermediate state are discussed in terms of superzone effects and s-electron reflection from the potential barrier at the interphase boundary.