Filter options on select_multiple for preceding question

Please, how can I filter options that appear in the next question based on what's was selected in the previous question. I'm using select_multiple and the choice list are same for all questions.
For instance,
Q1. What we're you trained on?

Q2. Which course we're you trained free of charge?
Only those selected in Q1 appear

Q3. Which course did you pay a fee?
Only those selected in Q1 appear. OR only those not selected in Q2 appear.
Please help.