Hi Stephane,
most of my customers go for the 3-system landscape on the BO side so it is aligned with the BW landscape being also 3-layered. Having a 3-2 layer landscape will have a small overhead when trying to connect to the three backend system from only two front end systems (like having different folders for each connection). This overhead is gone when if you go for a 3-3 landscape.
Do you really connect to BW Dev AND BW QA from your BO DEV? If yes, then a 3-3 Landscape will be better for you.
If you only go straight to BW QA (no need for BW Dev) from BO DEV, then no need for the overhead and 3-2 should be better suited for you.
Best regards,
Victor