Ο Τομ έχει μάθει να κολυμπά από όταν ήταν τριών χρονών.

I have done my best to correct this on tatoeba. The translation should be something like “Tom learned to swim when he was three years old.” I always hesitate to be definite about Greek, since I am not a native speaker, but the number word in this sentence is ‘three’, not ‘two’, and the ending phrase refers to age, not years ago.