1- (By) this time next week, we will have just arrived in Italy. (Is 'by' is necessary in this sentence?) It's possible with or without "by" but I would delete "just".
2- I will have waited for you until 5 pm. (The process of 'waiting' will last until 5 pm)
3- I will have waited for you by 5 pm. (I will wait for you 'no later than 5 pm or before 5 pm')
2 and 3 are incorrect.
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