Can I drive without being insured on that car?

So I am a 16 year old driver. my family has three cars. a lexus GX470, a Toyota Land Cruiser, and a saturn sky. I am insured on the Toyota and not the saturn because the difference insurance increases. ($75 compared to around $250) Can I drive the saturn and the lexus without being insured on those cars? And if so what are the limitations.