Prepare the coconuts by chopping off the bottom so that it can sit level. Then chop off the top so that the dome of the nut sticks out a bit. With one chop cut the top of the dome and let the juice come out into a bowl.
Scrape some of the meat, about one cup's worth. Strain and measure out the coconut juice, you will need 2 cups worth of juice. You should also taste the juice. If the juice is sour you will have to get new coconuts.
Mix the agar into the coconut juice in a wok. Let the agar sit for 10 minutes. Begin to heat the juice and add the sugar and salt. When the juice is bubbling, simmer for 10 minutes stirring constantly. Add the coconut meat and cook for 5 more minutes. The juice should be clear and slightly syrupy in appearance.
Divide the juice and meat between the 3 coconut shells and place in a refrigerator. Chill the coconuts completely before serving.