What everyone else has said is correct about the spot throw-in.
From a coaching stand point I'd add that one foot on or over the 3 foot spot, can cover a lot of floor without a violation.
What I look for to judge a violation is a step to the side and a crossover and step with the other foot. Unless that initial step was a small one, less than 18 inches, both feet have left the 3 foot space.
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