The only feasible conversion between .cht and .ch3 files would be to saving Harvard Graphics vector graphics (.cht) and to another Harvard Graphics chart/image (.cht3) files. Probably can be done through standard working process within the program and using File ► Save as.. function.
Convert Harvard Graphics vector file to Harvard Graphics v3.0 chart or drawing.
We have found one software suitable in our database for this conversion.
Microsoft Windows software - convert cht to ch3 on Windows
A data analysing tool
Convert from cht file format
The table with program actions contains information about what each program is capable of doing with their files. This may be a good pointer for further searches for certain file conversions and help you find the converter you are looking for if our search result is insufficient for you. However, it is far from perfect and may sometimes show results which are not really usable because of the nature of the software handles the files. So a conversion that appears as possible, just because of matched actions, may in fact not be possible.