As far as I know they all have a multi city search function. I prefer Google flights. Sometimes Matrix which is owned by google.Skiplagged.com is an example of a site that will search for multi-leg trips for cheaper prices.
Most of our flights are international .
AA is all screw up right now. Do not count on the flight you book actually happening. I booked Nashville-Phil-Paris. A quick trip with 1.5 layover at PHL. AA changed it to BNA-Lga(collect bags at LGA and take cab to JFK). Check in and security again at JFK. Fly to Paris. Original trip was about 12 hours. The new flights are about 19 hours.
Yesterday I cancelled and rebooked on United.