Ok - i think you mean when the 3rd party JAR is custom and not in the Maven repository. In that case...
You can place the JAR in your local Maven Repository and then reference it in your POM file. For example, to resolve myCustomJAR_1.0.0.jar within an AEM service, the myCustomJAR_1.0.0.jar file must be located in the Maven repository. You can upload the myCustomJAR_1.0.0.jar to Maven by using this Maven command:
mvn install:install-file -Dfile=C:/plugins/myCustomJAR_1.0.0.jar -DgroupId=com.foo.reports -DartifactId=jrurbersrt1.0 -Dversion=1.0 -Dpackaging=jar -DgeneratePom=true
Notice that you must specify the location of the myCustomJAR file in the -Dfile argument. Also notice that you specify the Dgroup and DartifactID values. Once you upload the JAR to Maven using this command, you can reference this JAR file using this Dependency.
<groupId>com.foo.reports</groupId>
<artifactId>jrurbersrt1.0</artifactId>
<version>1.0</version>
<scope>system</scope>
<systemPath>C:\Users\scottm\.m2\repository\com\foo\reports\jrurbersrt1.0\1.0\myCustomJAR_1.0.0.jar</systemPath>
</dependency>
Notice that scope element is system and systemPath references the location of the JAR file in the repository. (This dependency is used later in this article to build the AEM service)