Bonjour,tout d'abord il faut que tu saches que since = depuis et for=pendant Voilà,je pense qu'il faut reformuler comme cela :
a)i'm reading for three hours I started reading at 1 p.m. It's 3 p.m. and I'm still reading.
i'm reading since 1 pm. It's 3 p.m. and I'm still reading.
b)it is raining for 3 days because it started raining on saturday it's monday and it's still raining .
it it's raining since saturday today is monday and it's still raining .