A 7.5 watt heater should be fine for the 3g tank.
Nice that it has a filter and light!
The heater shouldnt melt the plastic, it wont get hot enough
to.
This tank because it is filtered is going to go through a cycle - the conversion of ammonia to first nitrItes and nitrAtes.
Heres an article with more info:
http://faq.thekrib.com/begin-cycling.html