are all these three sentences correct ?
-I will be travelling by plain and I will arrive at the airport at 8 pm
-I will be travelling by plain and will arrive at the airport at 8 pm
-I will be travelling by plain and arrive at the airport at 8 pm

Question

Answer

Travelling by plane = flying.

You can travel across the plain (flat land) by plane, car, bus or horse. Possibly camel.

Plane.
Plain.

But the sentences are all fine. Possibly the second one, as SuKi says, is the most common.

Answer

Apart from the misspelling of 'plane', yes, they are. The second one is the most frequently used form, omitting the subject but repeating the auxiliary verb.

Answer

I think all are right except for "plane".