In-text:
Rundle, Booysen and Dube (1990) states… or at the end of the sentence ... Online messages can be interpreted in various manners (Rundle, Booysen & Dube, 1990).
Reference list: Rundle, D., Booysen, H. and Dube, P. (1990). The interpretation of online messages. Timbuktu: Inside Publishing.