1. 완수하다 is used when you finish or complete some tasks or missions that are given by "others". So it means you finished your responsibility to do something. 2. 완료하다 means just To finish or complete. It is just a neutral word with no special feelings. 3. 완성하다 is used when you finish or complete something that you have been doing for some period of times. So it may be able to imply some process you've been through for doing that. It's used normally used for like finishing cooking, putting the puzzle, and any other things you made with putting efforts in it. Hope this is helpful for you!

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