I came from a domestic quad, so I was confused when people talked about the Big 3, too. It's a performance kit that EHS sells. A full exhaust, air intake lid (for more efficient air intake), and a fuel programmer (to add fuel so the engine doesn't run lean with all the new air). I think most people consider any bike with aftermarket intake, exhaust, and programmer a Big 3 bike, whether it's EHS components or not. But I could sure be wrong about that.
08 700XX (Gray, L8ton Racing Stealth Bumper, 13 tooth, 21" Razr2 fronts, stock rears, Precision Pro stabilizer, Fox Evols and Podium Xs, Teixeira Tech stock-length upper A-arms, Teixeira Tech stem, L8ton Racing 9/16ths sway bar, ProTaper Contour bar, Shock & Vibe clamps, EHS lid & programmer, HID headlights)
08 Outlander 800
08 Yamaha Big Bear 400 4x4