On my A7V8X, I get a nice long red error message on the right about not supporting more than one slot at DDR 400.
I haven't tried 3 slots, but I believe it is limited to 266 with 3 slots. I would listen to what the Asus manual says.
Max: 1 slot for DDR 400, 2 slots for DDR 333, 3 slots for DDR 266.
I am running 2 slots DDR 333: 2 x Corsair PC3200 DDR 512MB CMX512-3200C2 running at CAS 2-2-2-6.
The performance is almost always worse at DDR 400 than DDR 333, so there is no reason to bother trying to run at DDR 400. The Athlon XP at 266 FSB can't use more than a little bit above DDR 266 (because of overhead), so DDR 333 has a slight advantage over DDR 400. Unless one is using integrated graphics, DDR 400 isn't going to offer any advantage. It isn't the bottleneck. If you overclock past 160FSB one starts seeing an advantage with DDR 400 over DDR 333. However, most people are not having a lot of luck overclocking the A7V8X. Hopefully, newer BIOSes will fix that.
