Skip the 5D3 and go to the 5D4 if you have the funds. It is almost always the situation where skipping a generation as you upgrade is a better move.
The 5D3 is a great improvement over the 5D2 for sure, but the 5D4 goes beyond that yet.
5D4 over the 5D2 (some of these the 5D3 has, but in many cases, the 5D4 extends the functionality):
- Dual slots, faster SD over the 5D3
- Flicker control
- Touch screen
- Wifi/FTP/Web Services
- Great AF system, 5D3 had something close to the 1DX, the 5D4 has something close to the 1DX2
- New sensor, greater low ISO DR
- Cleaner high ISO
- 30Mpx
- Near dark AF
- GPS
- Better rear LCD
- Faster burst, longer bursts
- Better sealing
- HDR mode, Multiexposure Mode, High ISO Merge mode
- Edit raw in camera
- Better AWB
- Lack of banding when pushing shadows
- Generally better colors (IMO)
- DPAF for live view
- Better video
- AFMA, with 2 adjustments per telephoto lens
- Better menu structure, My Menu with multiple menus
- Button customization
- Spot AF
- Orientation linked AF point
The one and only advantage the 5D2 has over the 5D4 is the old-school LED per acquired AF point in the viewfinder vs the LCD overlay with a black AF point.