3. The seller has 10 melons and scales that can determine the total weight of any three melons in one weighing. How can you find the total weight of all the melons in six weighings?
3. Let's number all the melons: $D_{1}, D_{2}, \ldots, D_{10}$.

We conduct six weighings:

1 - melons $D_{1}, D_{2}, D_{3} ; \quad 4$ - melons $D_{1}, D_{2}, D_{4} ;$

2 - melons $D_{2}, D_{3}, D_{4} ; \quad 5$ - melons $D_{5}, D_{6}, D_{7} ;$

3 - melons $D_{1}, D_{3}, D_{4} ; \quad 6$ - melons $D_{8}, D_{9}, D_{10}$.

Let $S_{1}$ be the total mass of the 1st, 2nd, 3rd, and 4th weighings; $S_{2}$ be the total mass of the 5th and 6th weighings; $S$ be the total mass of all melons. Then $S=\frac{S_{1}}{3}+S_{2}$.