first time

[Anonymous]

1-This is the first time I am writing a letter to him.
2-This is the first time I write a letter to him.
3-This is the first time I have written a letter to him.

In which case:
a-I am writing the letter and haven't finished it yet.
b-I have finished writing it.
c-One can't tell.

[Rusty]

1-This is the first time I am writing a letter to him. (a)
2-This is the first time I write a letter to him. (Incorrect).
3-This is the first time I have written a letter to him. (b)

In which case:
a-I am writing the letter and haven't finished it yet.
b-I have finished writing it.
c-One can't tell.