How legally binding is a triple dog dare?:
there is NOTHING on the planet more binding than that of the Triple Dog Dare.  Example:

Thong-Wearing Men Arrested at Wal-Mart 
Thu Jul 29,10:59 AM ET
AP

SCOTTSBLUFF, Neb. - Two men who were arrested for walking through a Wal-Mart while wearing women's thong underwear blamed the stunt on a "triple-dog dare," authorities said. 

The men, ages 35 and 36, bought two pair of underwear at the store Tuesday, went into a bathroom and came out wearing only the thongs and T-shirts, police said. 

Witnesses said the men walked through the store and out to their car. 

Police caught the men in the parking lot, and reviewed a surveillance tape before arresting them for public indecency and disorderly conduct. 

When asked why they were wearing thong underwear, one of the men said a friend "triple-dog dared" them. They will not be prosecuted, authorities said.