Reasons :
1. first of all its Laptop if im not mistaking , you really should not expect constant 60fps from crysis 3 running on a LAPTOP
2. your specs are weak for running Crysis 3 at constant high FPS
3. Crysis 3 V1.0 has lots of problems , including Physx . update the CRYSIS to version 1.3 to solve the performance issue ( that worked for me )
4. this game has FPS cap . google for console code to remove the fps cap .
5. this game is so intensive even guys with strong PCs dont expect much from it . turn down your settings . try all settings to see which one gives you best quality with good FPS
6. if all these stuff didnt work . you have your last shot . its not that safe i think . some people dont recommend at all . try HYPER MATRIX tool , this tool can give you huge FPS boost but monitor your CPU & GPU temp so you wont damage your Hardwares .
good luck