You can download Processing3 from the usual site http://www.processing.org and you can find the recommended setup information at https://github.com/processing/processing-android/wiki but a number of people, myself included, seemed to be having trouble getting it to work.
After some digging on the forums I did manage to get it working so I thought I would summarise what had to be done.
1) If you have Android Studio or Eclipse installed, locate the Android SDK folder otherwise you will need to download it (SDK only) as described in the wiki above.
2) Not sure if this is 100% correct but from the wiki you will need to load API 10. You can use preferences from Android Studio or run the android app in the tools folder to do so.
3) I did add the variables in the .bash_profile but that did not work for me. Further digging led me to this post : http://forum.processing.org/two/discussion/12665/android-sdk-could-not-be-loaded
- open processing preferences, note the path of preferences.txt, probably
obviously <your name> needs to be replaced by your user name.
- Close Processing
- Edit preferences.txt and add the following two lines at the beginning
That seemed to do it for me. Haven't tried using an SDK > 10 even though somebody has claimed that it may work and haven't run a full test - but not getting the error must be a good start.