Well, 3dfx SLI and nVidia's SLI works very differently.
3dfx SLI worked by grabbing all the odd and even scan-lines and one GPU will do the processing on all the even scan lines, the other card did the odd scan lines.
Aka. 3dfx SLI was "Scan Line Interleave" where-as nVidia's is "Scalable Link Interface".
nVidia and AMD on the otherhand might have one frame on one GPU, another frame on the next or Split the scene up into two halves with each GPU doing their half of a scene. etc'.

Basically all nVidia got out of 3dfx SLI was the technology short-name. :P

Well, they didn't exactly have the best track record besides the Voodoo 1 and Voodoo 2.
Voodoo Rush, Banshee, Voodoo 4 and 5 all bombed or had bad-points.
The Voodoo 3 was only 16-bit (24bit) in a world that was moving to 32bit.
The Voodoo 5 6000 with it's 4x VSA100 chips could have been a beast, but benchmarks with that card against the Geforce 3 wasn't exactly favorable for 3dfx, but that was early stages, still it would have had a feature set that would have been behind nVidia and ATI.

Then you have the Geforce FX, nVidia's worse graphics card line-up besides the NV1 which wasn't even compatible with rendering what became the standard polygon.

So, really, the talented people at 3DFX (I'm not denying that they aren't talented!) had more products that did poorly than what did really well, nVidia was just aggressive with it's stupidly short development cycles which helped lead 3dfx to it's demise way too early IMHO and could have compounded why the Geforce FX was so horrible in many aspects.

Heck, I still have my Voodoo 2's, best retro GPU's of all time!

Also, my last comment was a more tongue in cheek comment. :P

The reason why 3Dfx fell behind was because of terrible management just like how amd is suffering from terrible management right now. 

SteamOS developed in collaboration with Nvidia

“NVIDIA engineers embedded at Valve collaborated on improving driver performance for OpenGL; optimizing performance on NVIDIA GPUs; and helping to port Valve’s award-winning content library to SteamOS; and tuning SteamOS to lower latency, or lag, between the controller and onscreen action. The collaboration makes sense as both companies strongly believe in the importance of open-platform innovation, and both companies are committed to providing gamers with a cutting-edge visual experience.”

http://www.vg247.com/2013/09/26/steamos-developed-in-collaboration-with-nvidia/

And part of that "bad management" came down to product execution. :)
AMD has faltered in that regard with servers with it's stupidly high profit margins and trickling that down to the high-end consumer space.
Also spinning off the FABS IMHO was a bad idea, they should have opened them up to other companies and produced chips for them to make 'em more financially viable whilst retaining highly-valued assets.

With that said, Low-end and Mid-range AMD CPU's are downright fantastic, my FX 8120 system has been terrific value for money once overclocked to 4.8ghz. - In no way however would I call it high-end though.
AMD GPU's however left much to be desired for the past 12 months where everything was stagnant with no product releases.
When geeks like myself don't have shiny new things periodically, bad things happen! :P

Spinning off fabs was actually a good idea seeing as how amd didn't have alot of money anymore to support expensive node transitions and plus that event led them to profit for the first time in the last 3 years. :P (No I am not joking about it, that tells amd that their in seriously bad shape.)

The reason why they didn't release anything in more than 12 months had to do with not having alot of performance gains too.