3D Modeling PC

ZBrush
-- uses CPU only, no video card acceleration. Needs highly threaded. Think i7 not i5 for CPU. (update: also think good aftermarket CPU cooler like Hyper 212 EVO or Noctua NH-U14S)
-- Any video card or integrated video from CPU. Video card is not used for rendering.
-- Uses lots of memory, Think 16BG min. Complex models go larger.
-- Disk Space discussed below.

Maya
-- Maya needs the same class CPU as Zbrush, highly threaded i7 best.
-- Maya uses the video card to accelerate some features. Unfortunately you need to make sure the video card you buy will work with the drivers used to accelerate rendering. That limits you to the expensive Quadro or AMD firepro version of video card rather than the same hardware sold as a gaming card. http://usa.autodesk.com/adsk/servlet/syscert?siteID=123112&id=18844534&results=1&stype=graphic&product_group=19&release=2015&os=8192&manuf=all&opt=1
--memory 16GB+
-- disk as below

3D Max
-- Make sure win10 is supported, it might still be win7 only.
--- CPU same as above
-- VIDEO card is used for acceleration, both AMD and Nvidia, again you need the firepro or quadro version. Double check if the nvidia CUDA support is better than the AMD support. I don't know and didn't google it.

DISK-- If you are doing work you can't lose then go with a RAID 1 array (simple mirroring). You want two NAS rated disks in your case, and can use either Intel drivers (RST) or microsoft's solution for mirroring. Suggest you get two 5TB-8TB spinning disks in a case that can hold at least 5 disks, so you can have a boot drive and later add another pair of large raid spinning drives. An external raid array is possible, make sure you use eSATA if you go external, you will not like NAS performance. You also need a good backup strategy. Backing up to an external disk periodically is a good idea. Do NOT have the backup drives plugged in all the time -- too easy for a virus to wipe both your drives and your backup. Do get a minimum 240GB SSD for your boot device and for product installs and for any workfiles, 500GB would be better (SSDs tend to support faster real world data rates as they get larger).

Update: Forgot the case. You will want good airflow, and a good CPU cooler. I use a fractal design r5 (actually r4, you should look at the r5 which replaced and is better than the r4 ) which keeps everything cool and quiet. Your PC will be cranking, and the VIDEO card and CPU will need plenty of cool air. Case must fit your drives, CPU cooler and video card. Aftermarket CPU cooler will really help the CPU run at it's limit by keeping it significantly cooler than intel part with the bonus of much lower noise.

Update: Buyer note: we are in the middle of a very significant video card update. Do not buy a firepro or quadro card until they are based on the new Pascal hardware for nvidia or Polaris for AMD. You want the much better power performance and, for Pascal, much better total performance.