My answer still valids for your new question.
Like if there are 3 companies selling "red widgets" I should only pick one company to advertise?
You can pick all three as you like to test which one will give you the best conversion.
To do so, you can create the same or slightly customized versions of ad for each company.
The ads will have different destination URL.
Dont forget to remove the check mark on "Automatically optimize ad serving for my ads", so that each ad (or each company) will be displayed equally.