There are many options to add more fans to your system.  Depending on which setup you choose, the required hardware then will be sourced to match.  But one thing for certain, top case fan is factory installed (not optional), it has to be there unless your machine had been altered. Front case fans are optional.  The upper hard drive cage should have fan mounts for a 92mm fan.  Installation is easy, just drop the fan in the drive cage and secure 4 rubber mounts to the eyelets (holes)  Bottom front fan will require a proper fan adapter, which can fit a 120mm fan.  The adapter should have 2 inner guiding latches and 1 outer snap lock. Beside physical fan installation, you may have to provide the power to each front fan.  The top case fan already has a fan header on motherboard, located next to CPU fan header. There are several posts in this community regarding cooling mods.  Search for them and decide what best for your fan upgrade plan.  Hardware specs and usages preference will determine fan types as performance fans can be loud but it will keep your system with effective cooling.  PWM fans can be added and operating together with your existing fan as demands and keeping system quiet when idle. A picture of your current machine and machine specs can be helpful as it can gather more suggestion input from the community.
Hi, I've cracked open the case of my Dell XPS 8930 but noticed that there isn't a front, top case fan there. There doesn't appear to be any way to screw on a fan there. Anyone here familiar with how it would attach? 
XPS 8930, how to install top front case fan?