It depends on (1) the effect size you expect to observe, and (2) the test you plan to use.

@Abdullah Jan, it depends on some factors. You may wish to throw more light on the test you plan to use and the effect size you expect as pointed out by @Blaije Tomkins. In addition, if the population is known, you may wish to use Taro Yamane sample size determination formula or use online sample size determination software or sample size determination table (you can find it online).

It will depend on study design, effect size and test statistic to be used. Generally, if just a cross-sectional study: when population size is know, you cn use Yamane formula... if unknown, you can use sample size formular for cross-sectional study. Thereafter, you will need to know the proportionate size of the 3 universities....and calculate the proportion of total calculated sample size that you would pick from each of the universities.

Thank you

Further readings:

https://pmc.ncbi.nlm.nih.gov/articles/PMC3775042/pdf/IJPsyM-35-121.pdf

https://pubmed.ncbi.nlm.nih.gov/32295935/