Pedal always relates to bicycles, pianos, organs, boats, looms, sewing machines, and other machines. The pedals are the foot-operated components. The word also functions as a verb meaning to operate pedals. Its participles are pedaled and pedaling in American English. Outside the U.S., they are pedalled and pedalling.
Peddle is a verb meaning to sell or to travel about selling goods. It often refers to the sale of illicit goods.
And petal is easy. It is always a noun referring to one of the colored parts of a flower surrounding the reproductive organs
Examples
Pedal
Bikers might choose to pedal into the artsy town of Sausalito, grab a drink or lunch and then return to the city via the ferry. [Dallas Morning News]
I looked up to see a mulleted giant pedalling past, and, sure enough, he was wearing real wooden clogs. [Telegraph]
Just 23 minutes of pedaling could boost health if done regularly, the authors say. [Chicago Tribune]
Peddle
The clampdown on gun-toting dealers and the crack they peddle is officially known, disconcertingly, as “pacifying” the favelas. [New Zealand Herald]
Earnest pundits and politicians peddled the myth that the killing of teenage girls by their fathers and brothers could happen to anyone. [National Post]
Petal
The scientists marked a lily bud with dots along each petal’s edge and “midrib” or central vein. [BBC News]
Their children frolic in the fallen pink petals, and National Park Service employees scold the naughty ones who pluck a fresh souvenir. [Washington Post]