Correct PHP-code for showing single group randomized audio

Question

I'm trying to randomly assign one of three different audio clips to a single group of participants. I see a PHP-code for two groups but not for a single group. Which PHP-code do I need?

Comments

I am not sure, if I get your question right. Do you have a single experimental group, and present them with 3 audio clips in a within-subject design? Or do you have a single group of respondents and want to split them into 3 experimental groups to present each one with an audio file (between-subject, one audio per respondent)? Or do you maybe intend something entirely different?

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I have a single group of participants who are listening to one audio then fill out the survey. That part is done. Once they have taken the first survey, I'd like to split them up into 3 different groups, randomly assigning them one of 3 audio clips. Afterwards, they will fill out another survey. Essentially, I'm testing within the subjects and between 3 groups. I hope that makes more sense.