Question

How l can write a brief summary about measurement for 3rd grade

Answer 1

The topic "measurement" is very, very broad.
You'd be best off by limiting your scope to a couple of familiar situations:

Weighing a piece of chocolate

Having your height and weight measured by the school nurse

Counting the number of coins in your pocket

Getting a rough measure of how well you have learned new material by checking how many questions you got right out of the whole test.