The slice selection gradient (G_{s}) should be created by a combination of y and z gradients.

G_{z} = G_{s} Sin 45^{o}

G_{y} = -G_{s} Cos 45^{o}

The phase encoding gradient (G_{f}) should be created by a combination of y and z gradients.

G_{z} = G_{f} Cos 45^{o}

G_{y} = G_{f} Sin 45^{o}

The frequency encoding gradient (G_{f}) will be equal to the x gradient.

G_{x} = G_{f}

Please note that the phase and frequency enocding gradients could be switched.