3D does not display every other frame to each eye. Rather a single frame is sent to the display device that has the images for both eyes encoded in it. In frame packed HDMI, 2 full resolution images are included in a single frame, the display device simply toggles between these 2 images at the cycle rate of the panels (and glasses) until a new frame is delivered. In side-by-side or top-and-bottom encoding, 2 half resolution images are stitched together and delivered to the display device as a normal image. The display device then splits the single image into 2, scales the 2 images to full size, and then toggles between them.
For a 24p source, you only have to have 96hz panels to take advantage of FI in 3D (since a 24p frame will be displayed 4 times, twice for each eye, an FI enabled display could display the original image once plus one inserted image per eye before the next frame arrived).
The new Epson and Panny technically have 480hz panels, but they only get full image updates 240 times per second (the panels refresh faster than the processor can copy a full 1920x1080 image to them, so it takes 2 cycles for a full image). At 240hz, a single frame from a 24p source can be displayed 10 times (5 times per eye in 3D mode). A 30p source would be 8 times (4 per eye), and a 60p source would be 4 times (2 per eye). As you can see, even a 60p source can take advantage of frame insertion in 3D on 240hz display devices.