Is that what he's doing (Netflix)?

My brother claims with his basic tariff (1 access) that he can stream on 3 devices at the same time

However, I know that it is not possible because I got 2 accesses, as I was always logged out of either laptop or mobile phone with 1 access.

No that doesn't work. At least not at the same time.

No. In the basic subscription only 1 stream is possible at the same time.

https://www.netflix.com/...p/planform