I haven't seen X-Men 3 in a while but, so I can't remember how the X-Men found out about Magneto's attack at Alcatraz. I can only think of a few explanations for how they found out and none of them satisfy me. A. Wolverine found out about Magneto's plans during his encounter with the brotherhood in the forest. This is unlikely since the most logical thing for the X-Men to do was alert the government about the attack so the military at Alcatraz would be prepared and they obviously weren't. B. The X-Men heard about the attack on the news. This is not likely because it would take the X-Men no less than 3 hours to travel across the US to stop Magneto in an attack that would have lasted for 5 minutes. C. Jean told Wolverine about the attack with her telepathy. This might have worked but, there is no evidence that Jean told Wolverine about the attack after Logan left the forest. If Jean told Logan while he was in the forest why didn't the X-Men inform the government when Logan got back to the X-Mansion.