The instantaneous voltages at three terminals marked X, Y and Z are...

Potential difference between $XandY = {V}_{X} - {V}_{Y}$

Potential difference between $YandZ = {V}_{Y} - {V}_{Z}$

Phasor of the voltages:

From phasor diagram the peak value of ${V}_{x} - {V}_{y}$ can be calculated from parallelogram law

${\left({V}_{x} - {V}_{y}\right)}_{peak} = \sqrt{{V}_{X}^{2} + {V}_{Y}^{2} + 2{V}_{X}{V}_{Y}\cos \left(60\right)} = \sqrt{{V}_{0}^{2} + {V}_{0}^{2} + 2{V}_{0}{V}_{0} \times \dfrac{1}{2}} = \sqrt{3}{V}_{0}$

$∴ {\left({V}_{X} - {V}_{Y}\right)}_{peak} = \sqrt{3}{V}_{0}$

${\left({V}_{x} - {V}_{y}\right)}_{rms} = {V}_{XY}^{rms} = \dfrac{\sqrt{3}{V}_{0}}{\sqrt{2}}$

Similarly ${\left({V}_{y} - {V}_{z}\right)}_{rms} = {V}_{XY}^{rms} = \dfrac{\sqrt{3}{V}_{0}}{\sqrt{2}}$