Synergy Stage 3 and Stage 4 lifts are both 4 link designs (Stage 3 is a 3" lift with short arms, Stage 4 is a 4" kit with long arms). Synergy doesn't sell a 3 link kit for JK's. One question, if you have a 2.5" lift currently, why would you even consider a 3" lift? Seems like a lot to spend for virtually no difference in ride height.
Their long arm kit isn't a bad kit (way better than say the Teraflex option), have quite a few friends that run it. It's pretty similar to the non-high clearance long arm kit that EVO sells though I don't think the arms are as stout having picked up both. Ride is much improved over stock arms with a 4 inch lift as you'd expect.
RK, Artec, Genright, and a few others sell 3-link kits for the JK but as Eddie mentioned, you absolutely run the risk of stranding yourself if one of your arms fails while you're wheeling. Bit more margin of safety with a 4-link/trackbar set-up. Probably not as big a deal if most of your wheeling is in parks. Much bigger deal if you're out west and the nearest help is many, many, many miles away over very rough terrain.