Four gyros are present on the Yohkoh spacecraft, one each in X, Y and Z and the fourth one in a 45 degree skew position. The IRU values are used by the IDL software to create the attitude (ATT) database.
In Sep-93, it was decided to switch to the X, Y and skew gyros (instead of X, Y, and Z) because of the high drift rate of the Y gyro.