$3 \mathrm{~g}=$ how many $\mathrm{mg}$ ?

Step 1 ：Convert grams to milligrams: 1 gram (g) = 1000 milligrams (mg)

Step 2 ：Multiply 3 grams by 1000 to convert to milligrams

Step 3 ：3 g = 3 * 1000 mg = 3000 mg

Step 4 ：\(\boxed{3 \text{ grams} = 3000 \text{ milligrams}}\)