Continue looking for other fellowships while you are on the NSF fellowship. These can be used either after the NSF runs out, or you can postpone a year of support and take the other fellowship instead.
Save your NSF-supported years for later on in your PhD. The NSF is an incredibly useful fellowship as it frees you from TA responsibilities for 3 years. However, it is important to use your support wisely and it is recommended that you save it for later in your PhD when you are focusing more on research and writing.
If you obtain an NSF early-on in graduate school it is recommended you postpone one year of support to be a TA for a course, especially if you have no experience. Being a TA is a rewarding and challenging experience in it's own right and will help you on a number of levels.
Look for summer internships! While the NSF provides substantial support over summers, doing an internship is a very valuable experience. Even if you want to go into academia you can learn a lot from an internship. Not only this, but it also provides you with a backup plan in case the academic job market proves challenging (it is).