Mass of box=1.5kg
Mass of diamond=3.57g
Total mass? With due regard for significant figures.

Remember: The operand with fewer significant figures determines the number of significant figures in the result.

The mass of the box is 1.5 kg, give or take a hundred grams -- we only know it is most probably between 1.4 and 1.6 kg. Therefore we will round the result to hundreds of grams, which means that only two significant figures will be kept:

1.5 kg + 0.00357 kg = 1.5 kg. -- the answer.

Note the difference: 1.500 kg means 1,500 grams, give or take several grams.

1.500 kg. + 0.00357 kg = 1.504 kg,
1.5000 kg. + 0.00357 kg = 1.5036 kg,
1.50000 kg + 0.00357 kg = 1.50357 kg.