3. A super balance uses weights of $1, 3, 9, 27, 81, 243, 729$ grams, with three of each weight. To weigh 2019 grams of iron particles, the minimum number of weights needed is ( ) .
(A) 6
(B) 7
(C) 8
(D) 9
3. B.

From $3 \times 729-2 \times 81-9+3=2019$, we know the weights used are: 3 weights of 729 grams, 2 weights of 81 grams, 1 weight of 9 grams, and 1 weight of 3 grams, totaling 7 weights. Among them, the weights representing the subtracted grams are placed in the tray containing the iron particles.