You can use Open Weather Map.
OWM has a weather forecast of up to 5 days. The use is free, but you have to register with OWM.
In the app you can then set a weather forecast in an interval of 3 hours (3 or 6 or 9 or 12 or 15 or 18 hours, etc. up to 5 days). Unfortunately only one setting is possible, but this should be enough for your needs.
For more information I recommend this post. It explains exactly what you have to do. Examples are also included.
If you still have questions, I will try to answer them.
