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ttt stands for "to the top". As posts age, they sink among other posts. It's an empty ttt message used to move a post up to the top of posts so Forum members see it and maybe respond.
There must be a FAQ on this somewhere but it means To-the-top. You may notice that when you reply to a post it gets put to the beginning of all of the posts. Posts tend to drop to the 2nd or 3rd page quickly so usually the original poster posts a ttt reply to get the post back on the 1st page.
I had to ask my wife when I saw it on one of her craft forums..
My girlfriend doesn't use forums so she has no clue about a lot of the terms, but there are sites with most of the acronyms. The one from other forums I used to see that got me was QFE, which I found was Quoted For Emphasis.