What is happening here?
In grade 3 mm, cm, m, km, g, kg, t, ml, l and dollars/cents come together, plus minutes and seconds. Converting almost always means multiplying or dividing by 10, 100 or 1000 (by 60 for time): going from the big unit to the small one the number gets bigger, from small to big it gets smaller.
Then come mixed amounts: 2 m 50 cm is 250 cm. They prepare the decimal notation (2.50 m) of grade 4.
What helps at home
Make the units tangible: a meter is one big arm span, a centimeter the width of a fingernail, a kilogram a bag of sugar, a liter a milk carton. Let your child estimate in the kitchen and while shopping before anything is weighed or measured.
Before every conversion ask: "Should the number get bigger or smaller?" A child who says that first mixes up multiplying and dividing far less often.
What can go wrong
The most common trap is time: 1 hour has 60 minutes, not 100. A child who writes "230 min" for 2 h 30 min has carried the decimal system over. 1 km = 1000 m and 1 m = 100 cm also get mixed up easily.
For money, an everyday check helps: 250 cents are 2 dollars and 50 cents, because every full hundred is one dollar. A cheat table on the fridge (m, cm, mm, km, kg, g, l, ml) saves nerves as long as it stays a support and does not replace understanding.