3 month Relative Strength measures a stock's price change over the last three months relative to the price change of a market index. It shows the relative outperformance or underperformance of the stock in that timeframe. Different benchmarks are used for different geographic markets which can be reviewed here.
It is calculated dividing the price change of a stock by the price change of the index for the same time period. e.g. A stock falling by 20% versus an index rising 20% would lead to a Relative strength calculation of 100 * ( 80/120 - 1) = -33%