When you refer to several sources by several authors in one in-text reference, list the sources alphabetically by author and separate them with a semicolon:
(Harrell, 2013, p. 56; Kalliala & Toikkanen, 2012, p. 67; Robinson & Breen, 2021, pp. 30-31)
When the same topic is dealt with by various sources, the text may include several in-text references:
The findings of the research on reading in the childhood are not coherent (Albright et al., 2004, p. 78; Gibson, 2011, p. 60; Smith & Wexwood, 2010, p. 56).
In the study by Smith and Wexwood (2010, p. 56), the number of books read increased, whereas in the study by Gibson (2011, p. 60) their number decreased. Albright et al. (2004, p. 78) did not find significant differences.