In many cases, they are interchangeable. 完成 has a sense of achievement and completion. 結束 means the process of something is finished/complete/over. 了 indicates past tense. 完 is a complement like 好, 到 -- -- complements go after verbs. EX:　吃完了(finished eating it), 做好了(it's done, and done well), 看到了(saw it, achieve the seeing of something)

There all have a same meaning. '结束' —finish '完成'—complete '完了'—over, which exists mostly in the oral.

"结束" in the normal is used to end a plan, class. "完成" is used to just end a order. "完了" is used to break a thing such as a plan, a order, the order and plan is not ended.

It is kind of done-->結束和完成。 screw up --> 完了。

1.完了，是指某事，某物，某些抽象的，或者某些具象的东西不再持续，不再继续的意思。 2.结束 表示某些事情，某些关系，通常是指物体，或者抽象事情，表示完了的意思，没有了，不再继续了。 3.完成 表示某些事情Done好了，FINISH。 4.SO. 所以完了>结束>完成