In German you might say "seit drei Monaten", translating word-for-word to "since three months", but meaning "for three months now". The author in this case is French, where saying "depuis trois mois" is perfectly fine (as far as I know).

English, where that construction sounds weird and at least needs some helpers around it to exist, is a bit of an odd one out. It is kind of odd that we can say "for three months" to say that something took three months, but we can't say "since three months" to refer to something that has been going on as of three months ago.