PGE, I have had the class work in groups to figure out how many trips around the equator equal a trip to the moon. This has worked great with Grade 5 and 6 classes. With a Grade 3 class I might use addition and/or calculators .

you need a globe, a ball that's less than half the size of your globe and some yarn. Have a few students come up and show the class where they think the moon should be. then talk about the Equator and the distance to the moon Up north we use metric but if the equator is 25000 miles long and the average distance to the moon is about 239 000 miles how many times would I have to go around the Earth to equal a trip to the moon.

http://ssrsbstaff.ednet.ns.ca/jcroft2/How%20farmoon.htm
Multigrade's idea for fractions and oreo's is great but I'd wait until after the next New Moon so the students can watch the moon "get Bigger" in the evening sky.