Triple green is an old saying among many equity traders who trade equities in US stock markets.
On old monitors, the instruments are displayed in green when they are net up for the day so far and in red when they are net down for the day at the moment.
When the cash indices S&P 500, Nasdaq 100, and Dow Jones Industrial Average all turn from red to green, the quote boards would effectively turning mostly green across the symbols listed. The visual effect was quite stunning.
Triple green simply means all 3 indices turning from red to green. Many traders on the short side are more likely to give up their short positions when that happened.