3. Hua Hua writes letters to Yuan Yuan with a ballpoint pen. When the 3rd pen refill is used up, she is writing the 4th letter; when she finishes the 5th letter, the 4th pen refill is not yet used up; if Hua Hua uses the same amount of pen refill for each letter, then to finish writing 16 letters, Hua Hua needs to prepare at least $\qquad$ pen refills.
$13$