Two 15', or 20', high speed cables in series do not make a 30', or 40', high speed cable. That said, 3D at 1080p/24 or 720p/60 at standard color depth should be doable with a high quality cable. The bandwidth required is well below the maximum bandwidth of HDMI.
I just bought the HDMI extender over two cat5e/6 from monoprice (Product ID 8008) and I have tested it over two 75' cat6 I also bought there from my Panasonic BDT100 to my Mitsubishi 3DA-1, as well as from my PC to the TV and it passed both HDMI Full HD 3D (in other words, HDMI 1.4) and 1080p/60 without any problems. I'll be using this to send HDMI from the living room to the bedroom, I'm going to buy another set since it works to send from living room to the kitchen.