Hi Rachel,
Yes, you can. If the jar file contains classes A, B, and C, each with its own main(), then you can run them individually just by using
java -classpath myJarFile.jar A
java -classpath myJarFile.jar B
java -classpath myJarFile.jar C
If there were a manifest, it could point to one of these three to run by default when the jar was double-clicked, but you'd still be able to run the three programs separately as above.