Each of these three help you to communicate information to students, but are distinct in how you use them.
Agenda items are a list of what's going to happen in a class on a given day. For example,
"Today, we're going to take a quiz, read pp. 103-105 and fill out a worksheet"
Task Items are a list of what students are supposed to do that day and are intended to be listed on their ClassReach account (and their parents') so they can check off each one when they're done. Also, perfect for an ungraded homework assignment (eg, "Read Chapter 1).
Homework is a graded assignment that would be listed as such on a given day and also added to the task list
In this example you'll see all of them, plus the lesson plan for the week.
Notice that "review for quiz" and "Read Ch 3..." show up as homework and a task.
Homework is a blue pencil icon
Lesson plans are an orange sheet of paper icon
Graded assignments (entered into the gradebook) are a green icon
Agenda is a blue clipboard icon
Handouts are a purple sheet of paper icon