How to find the corresponding class in clf.predict_proba()

As a rule, any attribute in a learner that ends with _ is a learned one. In your case you're looking for clf.classes_.

Generally in Python, you can use the dir function to find out which attributes an object has.

import pandas as pd
test = [[0,1,1,0],[1,1,1,0]]
pd.DataFrame(clf.predict_proba(test), columns=clf.classes_)

Out[2]:
         one       three         two
0   0.542815    0.361876    0.095309
1   0.306431    0.612863    0.080706

Just use the .classes_ attribute of the classifier to recover the mapping. In your example that gives:

>>> clf.classes_
array(['one', 'three', 'two'], 
      dtype='|S5')

And thanks for putting a minimalistic reproduction script in your question, it makes answering really easy by just copy and pasting in a IPython shell :)