Hi,
1. This traffic is terrible. We’re going to be late. By the time, we get to the airport, Bob's plane will have already arrived.
How about "By the time we have got to the airport, instead of "got to the airport"? Say 'get', as in the original sentence.
2. The traffic was very heavy. By the time we got to the airport, Bob’s plane will have already arrived. You can't mix past with future in this example.
How about "By the time we had got to the airport", instead of "got to the airport"? You can't mix past with future in this example.
3. I don’t understand how those marathon runners do it! The race began more than an hour ago. By the time they reach the finish line, they (run) ___________ steadily for more than two hours. I don’t think I can run more than two minutes!
In this question, the book says that the correct answer is "will have been running". Yes, I agree.
But how about "have been running"? You could say this if you were speaking generally about all races. But not here, because you are talking about the specific race that began an hour ago.
Best wishes, Clive