25mm is the 1/4 wavelength in air. In the substrate, the value will be smaller.
If you would have field only in the substrate, and no field in air, then the wavelength would be smaller by a factor SQRT(epsilon_r) exactly. In microstrip, depending on the line width, most field is in the substrate but some field is in the air, so that the effective epsilon_r is smaller than the substrate value.
AWR have a transmission line calculator called "TXLine" that will calculate the effective epsilon and the required physical length for 90° at 3GHz, for your specific substrate data and line width. Alternative: You could also use a schematic model and just simulate the phase of a microstrip line segment.