|Helpful. Used about objects, but also about people who try to help. As in, Please let me know if there’s anything I can do to help.
|Convenient. Usually used about an object but also about a route:If you want to get to Sacramento, the bus is more convenient than the monorail. TTT
|Unlike the other two, 都合がいい is only used about situations:Whenever it’s convenient for you. Usually used when you’re trying to make plans with busy people. PPP