There is a \therefore symbol in standard (La)TeX, but it requires what's called the amssymb package. I didn't convert those extra font families for mimetex. But you can "fake it" using the -DNEWCOMMANDS="filename" switch on mimetex's compile line. See http://www.forkosh.com/mimetexmanual.html?options for a discussion. For your situation, the following line in filename seems to do the trick for me...
{ "\\therefore", NULL, "{\\Huge\\raisebox{-4}{.\\atop.\\,.}}" },
Mathematics tools
Therefore symbol
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