come about = to happen: How did it come about that everyone knows my decision?
come across = 1. to seem to be a particular kind of person: He comes across as an idiot but he’s really very intelligent. How did I come across at the interview?
2. to find something by chance: I came across this old jacket in my cupboard.
come along = 1. to make progress: If you work hard, you will come along nicely.
2. to go with someone else who takes the lead: I'll come along on the hike.
3. to show up; appear: Don't take the first offer that comes along.
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