3. Grandpa Zhang planted a row of pear trees, a total of 18 trees, with an interval of 3 meters between adjacent trees. Next to the first pear tree is a well, from which Grandpa Zhang fills a bucket with water and then walks to the pear tree to water it. When the water in the bucket is used up, he returns to the well to refill it...... Each pear tree needs half a bucket of water, and after watering all the trees, he rests on the spot. How many meters does Grandpa Zhang have to walk at least during the entire watering process? (10 points)
Grandpa Zhang has walked at least $3 \times 1 \times 2+3 \times 3 \times 2+3 \times 5 \times 2+\ldots+3 \times 13 \times 2+3 \times 15 \times 2+3 \times 17 \times 1=6 \times$
$$
(1+3+5+\ldots+15)+3 \times 17=6 \times 16 \times 8 \div 2+51=435 \text{ (meters)}. 
$$