COLOMBO, April 24 (Reuters) - A suspected Tamil Tiger rebel claymore mine tore into a civilian bus in the embattled north of Sri Lanka late on Monday, killing three people and injuring about 37, the military said. The passenger bus was travelling to the capital, Colombo, from Mannar in the northwest when it was struck by the mine, the media centre for national security said on its Web site. The blast was the third of its kind this month. As in the previous two claymore mine bus attacks, a Tiger military spokesman early on Tuesday denied the group's involvement, and said it was part of a government campaign to tarnish the rebels' image.