You will do 1 return only enter one T4 at a time. Once, one is entered on the bottom it will say " Add another T4" click on that to complete the next one and so on till you have all 3 entered then you will click on DONE entering T4's and continue.
There is no "add another T4" option that I can see. The field only allows for the uploading of one T4. I have to upload additional T4s under "other docs", which is fine, but I'm puzzled by the fact that the Employment Income field only allows the uploading of a single T4, as if no one ever works at more than one place per year. Every year, I have at least two T4 slips, and sometimes three. It doesn't make sense that the ability to upload more than one T4 has been taken away.