Data visualization on three factors I have three factors,

Income(from source A)，continuous variable
Income(from source B), continuous variable
Happiness index，continuous variable

Suppose I have 500 samples. My goal is to show the influence of both Income(from source A) and Income(from source B) on Happiness index. Meanwhile, I want to show the distribution of Happiness index. I think a boxplot on Happiness index is a good choice. I can use color to represent Income(from source A), however, how can I represent another continuous variable, i.e, Income(from source B) here?
What you can do is discretize happiness index using cut() and assign labels to it.
For plotting then you can use groupby boxplot which will show effect of both income sources on a particular happiness index bin.
Refer GroupbyBoxplot
If you want distribution of happiness index then you can try:
Color Scales