After thinking a lot, I found an "easy" way to do it :)
In my views,
- In MULTIPLE FIELD SETTINGS, I set it to display only 1 value, starting from 0.

Then I added the author field again, for 3 times with the settings above.
For each, I changed the Multiple Field Settings as below:
- author: display 1 value, starting from 1.
- author: display 1 value, starting from 2.
- author: display 1 value, starting from 3.
For the 2. and 3. authors I enabled the Rewrite the output of this field
option and this is the rewrite text for author 2: , [field_author_1]
and this is for author 3: , [field_author_2]
.
I also enabled the Rewrite the output of this field
for the 4. author and this is the rewrite text: ...
As a final step I added a Global: Custom text
to views and this the text:
[field_author][field_author_1][field_author_2][field_author_3]
It's done!
If there is only one author it displays Author 1
If there is two authors it displays as: Author 1, Author 2
(remember the rewrite:, [field_author_1]
).
If there is three authors it displays as: Author 1, Author 2, Author 3
And if there is more than three authors it displays as: Author 1, Author 2, Author 3 ...
(remember the rewrite for author 4:...
)