What happens to IP3 molecules after release from IP3 receptors?
All biomolecules are eventually broken down. This process is called turnover. Since inositol is a signalling molecule it is necessary to remove in order to terminate the signal, despite adaptive mechanism in the ER calcium channels. 

IP3 is generally dephosphorylated by a family of phosphatase enzymes called inositol polyphosphate phosphatases. After dephosphorylation, the inositol can be recycled into the membrane by phosphatidyl-inositol synthase. 

Reference:

The IUPHAR/BPS Guide to PHARMACOLOGY 1

1 I'm citing this website instead of a research article because apart from citing the original research articles, it also has other informative links about the enzymes and the substrate molecules. It is also from a very reliable source.