The field representation of the Caneschi-Schwimmer-Veneziano vertex, including ghost terms is formulated and the equivalence to the oscillator form is shown. Extending the fields of the vertex to superfields, the cyclic symmetric vertex of the three-Neveu-Schwarz string is derived. The BRST invariance is proven and the structure of the vacuum which is attached to the vertex is analysed in connection with the BRST invariance. The role of the picture changing in the string interaction is discussed.
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