How do I program mandatory blocks as opposed to optional ones in Rise 360?

To require one of three videos, I suggest you put the videos in a Storyline file, and insert that as a block that must be completed in Rise. 

• In Storyline, give them the option to watch any of the 3 videos.
• For each video, add a "Complete course" triggers that runs when the video completes.
• When you publish the Storyline file to Review 360, indicate that completion should be tracked via a completion trigger. 

To require someone to complete an interaction, insert a CONTINUE Divider block. Edit the CONTINUE's  Completion Type to require that the user complete the block directly above the CONTINUE button. (Note: another option is to require that they complete all of the previous blocks.)

For the acknowledgement, you could use a simple Checkbox List block, followed by a CONTINUE block that requires them to complete the previous block. That could look something like this: 

Another option would be to put the acknowledgement statement(s) in a Storyline block, as described in this post: TIP: Acknowledgment statements for compliance - Articulate Storyline Discussions - E-Learning Heroes