There are always maunufacturing tolerances which will account for small differences between phases. If the differences are large then it is time to ask questions as there may be a problem with one winding.
the 3 phase power tansformer load loss measured by 3 wattmeter method. the total losses is sumation of w1+w2+w3=total losses
but as 3 kimbs of transformer is identical but w1 w2 w3 are not identical why this happens
You talk about the ” Copper Losses “, and we know that losse for each coil is : I² x R, that means the value is depending on 2 factors. Assume now that R has the same value for all coils ( it's 100% theoric ) the values for W1, W2, & W3 are depending on the circulated current in each coil, so, even if the currents are the same the R1, R2, R3 haven't 100% the same value.