Converting from Guitar Hero (.sgh/.tgh)
Use this tool made by Goulart, GH3toCH, to extract all your GH3 songs automatically making them usable for CH. Be warned they will not have any metadata other than artist and song name. Backup your DATA/PAK folder and follow the instructions on readme.txt, or watch this tutorial made by Acai.
Converting from Rock Band (_rb3con)
Acai made a tutorial for this.
Converting from Phase Shift/Frets on Fire (Normal Files)
Some Phase Shift/Frets on Fire note charts are badly formatted, thus Clone Hero can't read them properly. There's a tool Srylain made called Midifix that can try to fix them, but it is only able to fix midi files, not .chart files, and sometimes it can't fix some. Just download it, extract all the files, and then drag and drop your Songs folder ONTO the MIDIFix.exe.