The only way I know of to allow a person to sign up in more than one session is if they already have credit for the first session...but that means they have credit for the whole activity.
When we have three things someone has to attend, we set up three activities.
Does everyone do the same three sessions? (so if five people sign up for Monday, they also sign up for Wednesday and Friday, but the people who sign up for Tuesday go to Thursday and Saturday?) If there are three distinct dates and they're the same, you could add multiple dates to each session, which shows the three dates in the messages (so the session dates would be July 3 from 10-2; July 13 from 8-12 and July 23 from 2-6. It would only send one message (as far as I know) but it might work.