There is experimental support for python3 in some modules, the status of these is:
svm : OK linear_models : ? cluster : ? covariance : ? grid_search : ? mixture : ? externals.joblib : ? utils.sparsetools : FAILS to compile
To generate python3 compatible sources for selected modules, run the 2to3 tool on the module:
2to3 -wn --no-diffs sklearn/$module
If you would like to help with porting to python3, please propose yourself in the scikit-learn mailing list:
https://lists.sourceforge.net/lists/listinfo/scikit-learn-general