variable: r_StereoDevice DUMPTODISK, REQUIRE_APP_RESTART
type: int
current: 100
help: Sets stereo device (only possible before app start)
Usage: r_StereoDevice [0/1/2/3/4]
0: No stereo support (default)
1: Frame compatible formats (side-by-side, interlaced, anaglyph)
2: HDMI 1.4 (PS3 only)
3: Stereo driver (PC only, NVidia or AMD)
4: Dualhead (PC only, two projectors or iZ3D screen)
100: Auto-detect device for platform

variable: r_StereoMode DUMPTODISK
type: int
current: 0
help: Sets stereo rendering mode.
Usage: r_StereoMode [0=off/1/2]
1: Dual rendering
2: Post Stereo

variable: r_StereoOutput DUMPTODISK
type: int
current: 4
help: Sets stereo output. Output depends on the stereo monitor
Usage: r_StereoOutput [0=off/1/2/3/4/5/6]
0: Standard
1: IZ3D
2: Checkerboard (not supported on X360)
3: Above and Below (not supported)
4: Side by Side
5: Line by Line (Interlaced)
6: Anaglyph

variable: r_StereoEyeDist DUMPTODISK
type: float
current: 0.02
help: Maximum separation between stereo images in percentage of the screen.

variable: r_StereoFlipEyes DUMPTODISK
type: int
current: 0
help: Flip eyes in stereo mode.
Usage: r_StereoFlipEyes [0=off/1=on]
0: don't flip
1: flip

variable: r_StereoScreenDist DUMPTODISK
type: float
current: 0.25
help: Distance to plane where stereo parallax converges to zero.

variable: r_StereoStrength DUMPTODISK
type: float
current: 0.5
help: Multiplier which influences the strength of the stereo effect.

variable: r_StereoSupportAMD REQUIRE_APP_RESTART
type: int
current: 0
help: Disables/Enables [0/1] stereo support on AMD hardware



The last one may be all you need if you just need the 3D options menu enabled, but you may also want to set r_StereoOutput and r_StereoDevice to SBS just to be sure. Also, with r_StereoMode, I've found that only Post Stereo works. I am also using a Radeon 5870. Hope this helps.