'Increases to ' is fine in the sentence, Anon, where '3 times its original length' is the ending size of the growth; the final size is therefore 3X (the trunk's original length is X). Your sentence is also OK, but '3 times its original length' is the extent of growth, so that its final size is 4X. )
New words, one handy idiom, and a 2-minute quiz — delivered to your inbox to keep your streak alive.