What are the outliers of this data set?

Pretty much.  There's no one single definitive mathematical definition of what is and is not an outlier (you can formally make mathematical definitions, but there is more than one way to do so and which one people use is subjective).  But by any reasonable definition, 3 and 123 will qualify here, and the others will not.
Depends on the course you are doing, as hh26 points out outliers might be differently defined depending on the situation. However in my high school statistics, it was the  ‘quartile +or- 1.5*interquartile range’ rule.

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