" I have done this since three years ago " is the right answer since i was three years old = depuis que j'avais trois ans it's a subject verb object since three years ago = depuis trois ans i don't know how to call it but it's not a subject verb object sentence ; hope this can help u :)

Bonjour Correction de la question posée : "Comment comprendre cette phrase ?" A bientôt...

>>>"I have done this since three years ago" = "J'en ai fait pendant trois ans" (and I have stop) But here it's "j'en fais depuis trois ans seulement" so it's more like "I do this since only three years" (and I still make this!!) Since three years old = Depuis que j'ai trois ans (don't use the past) Depuis que + present >> depuis que je te connais / depuis qu'il habite ici...