In IGMPv1 and IGMPv2, receivers would only join a destination group, and traffic from any source sent to this destination was going to reach the receivers. Version 3 introduces support for Source Specific Multicast (SSM), which means that receivers will only get the traffic sent by a specific source. This means that when receiving IGMPv3 JOINS, a router can create specific (S,G) entries in the multicast routing table (mroute). With IGMPv1 and v2, a router can only create (*,G) entries when it receives IGMP JOINS.
When joining an IGMPv3 group, a host can also specify the sources it wants to receive traffic from (or those that it doesn’t want to receive from). The process is similar to IGMPv2.