a flatspin and a rodeo aren't the same thing but they're thrown in a similar way. a rodeo is completely inverted and a flatspin isn't.
check our andrea hatveit's blog for trick tips on this. he starts with a backflip and slowly leans to the right taking off each time which eventually becomes a rodeo. so a rodeo is sort of like throwing a backflip while leaning to the right and spinning to the left, all at the same time. a flatspin is more about throwing the right shoulder forward rather than flipping backwards. its hard to describe without looking at a video at the same time.
PS: a flatspin 3 is very rarely a true flatspin. its almost always on the rodeo axis rather than the flatspin axis...dont ask me to explain why though.