Sony have always been forthcoming with information about which of their online servers are going to be turned off. Bad news for Resistance 1, 2 and 3 fans as you have until March 28th to play online, after this date the online features; including Resistance 2 and 3's online co-op modes, will be no more.
Once more there is no reason given for the online features being turned off, but the new generation of consoles and lack of online activity for certain games must play a factor. Activision has recently pulled their Marvel games from digital platforms and Sony's latest server shutdown does suggest that a trend is starting to happen (whether it is for licencing or monetary reasons).
All the Resistance games have local co-op that will continue to work, but I for one will miss the 8 player online co-op of the second game and the impressive 2 player online co-op of the third. It is not only the co-op friendly Resistance games that have given up the online ghost. In May, Gran Turismo 5 will also be having its server severed.
Source: Uk.playstation.com