One advice:
Don't do it!
The BL3500 isn't designed for that task and way too underpowered. I've heard the BL3500 as center, tried 2 x BL 4000 and finally sold everything and got me a BL7-4.
In a 5.1 Setup, the voice, conversations,whatever comes from the front, the surround from the speakers surrounding you, so the voice should never disappear in all the effects, so you need a strong & powerful center, which the Beolab 3500 isn't.
Take your time, save some money, go for the right one, you won't regret it
Stephan