helpful, convenient

(やくにたつ) 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.
便(べんり) 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
がいい(つごうがいい) 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