1. I just can't believe it!

2. I simply can't believe it!
3. I exactly can't believe it!
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Thank you very much for your help.
You said #1 mean #2. Then, what about #3? What is the meaning of #3? What is the difference between #1 and #3?

1 and 2 are correct and mean the same thing, yes.

3 is incorrect -- "exactly" doesn't work in this expression.