Welcome to the forum, Emma.
I sympathise with your problem, but you did accept the job. If your job is to follow the book, then that is what you are paid to do.
Instead of thinking of an lesson of three hours. think of three lessons of an hour each. Stifle your creativity and follow the suggestions in the Teacher's Book. I don't know this course, but TBs generally give sound ideas for a range of activities in 45- or 60-minute chunks. You can have a two-minute fun activity after the first and second hours if you like.