Scheduling Methods:
A time slot for each, and the student pick up whatever they want of that subject until they all get finished. Example: Time Slot for science: Each time they could pick one of the following ( Explore the World of Physics, Bite-size Physics, Instant Egghead Guide to Physics + 7 go-alongs) until they all get finished.
Another Idea schedule a different one for different day of the week.
If you want each option scheduled each day, divide the science hour into 20 minute sections for the 3 options; or if you have half an hour for science 10 minute sections.