Well, lets take a look at the formula first.
1 yard is equal to 3 feet.
So, lets keep going...
2 yard is equal to 6 feet.
and lastly,
3 yards is equal to 9 feet.
(See the pattern? For every 1 yard, there will be 3 feet)
So, to answer your question, no. 3 yards is not equal to 21 feet, it is indeed less than 21 feet.
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~Hope this helped :)