The three mean essentially the same thing. If there is the subtlest difference in nuance, it is an implied suggestion of certainty about the arrival: 2 conveys the most certainty that he will arrive; 1 implies the least certainty; 3 is intermediate in certainty between 2 and 1.
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moon7296If there's any differece, how is #1different from #2 and #3?It is simply not possible to say which is more certain. Nor is it possible without far more context to say exactly what was in the speaker's/writer's mind. For example: