I've been doing insanity for 3 days now and my body is so sore. what can i do ?

See below: Back off 50%, then add 10% each week.

Slow down!: It's normal and expected to be sore after starting any new exercise regimine, especially an intense one . Take some motrin, advil etc a/o tylenol, (acetaminophen) ice your sore areas. Take a few days rest and start again, but go at a pace that you can tolerate. Just because the tape tels u this must be done, some people will respond differently and may have adjust.