A bar chart would be appropriate for X1; I would recommend histograms for X2 and X3.

While you could perhaps guess outliers based on the graph, it would be better to perform some calculations. There is no absolute requirement here; however, a good rule of thumb is to look for outliers >1.5 times the interquartile range from the 25th and 75th percentiles. The IQR is equal to the 75th percentile minus the 25th percentile.

I will leave the work as an exercise for you...but as a hint, X3=5.1 is the outlier.