Need some clarification please - do you mean Filipendula, and if so, which variety? If you do mean Filipendula, F. rubra's height is listed as 1.5 metres, and some of the varieties get between 1.5 and 2.5 metres. I'm also not sure what you mean when you say the flowers 'normally compact and small', are '3 feet tall' this year, do you mean the flowers themselves are 3 feet tall and also large, or the plant is 3 feet tall? The flowering stems on these plants are taller than the surrounding foliage, usually, so a flowering stem of 3 feet wouldn't be unusual. Any chance of a photograph?