If you're doing three fans on a single header, that should be fine. Just make sure their total current is less than 1A, which is the case for most 3x PC fans, and the A12x25 is less than .2A iirc, so you can put more than three of those on a single header. It's safe to use a splitter in that case. I still wouldn't do more than three on one header though.

If you're looking for a hub to run more than three on one header, I use the DeepCool FH-10 and it's pretty nice. Any 3-way fan splitter should work fine. Noctua has some, or you could use something like this.

Edit: the specs page says they're 0.14A, so you could in theory put up to seven on a 1A header but that's kinda risky imo.

Don't forget that with 9 fans, you will need a many-slotted fan controller. I think you may also need an rgb controller - I'm not sure which one is preffered with EK Vardar fans, but maybe the Corsair RGB Hub would work well.

Is the reason for all S radiators that if you go bigger on the bottom of the case, your vertical mount won't fit? If you want to go purely for performance, you could ditch the vertical GPU mount and just mount it as "normal" and put a 360X radiator on the bottom of your case, increasing the overall cooling capacity of your system significantly.