I have 3 fans in my case and need to speed them up as my computer is overheating.
How do I check/change the speed of my fans?
As was already stated, most BIOS setups include a "Hardware Monitor" item that will show you details of installed fans and fan speeds, voltages, etc.
But they also might include a "performance vs. silence" type of setting, primarily regarding the CPU cooler fan but possibly for the other case fans as well. Of course be prepared for increased noise from the CPU fan if it now spins at 2700RPM instead of 1800RPM or 1200RPM.
Many motherboards (e.g. from ASUS) have an "energy saving" piece of software (ASUS calls it "EPU-6 Engine") to go with it which monitors temperatures, voltages, fan speeds, etc., and can adjust the voltages sent to all the fans so that they spin up or down variably depending on "need". This goes along with the BIOS setting to enable this software control when Windows is running.
Some case fans have an optional resistor installed on the cable going to the fan header on the motherboard, to FORCE a lower speed (typically this is then fixed at that lower speed). Obviously this is an installation option and can't be undone unless you remove the resistor, so while it may be quieter spinning at a lower speed it's also going to produce a lower CFM airflow result, which will obviously reduce the cooling capability of that fan.
As far as actually SEEING (i.e. while under Windows) what your fans are doing, that again depends on your hardware and whether or not the onboard I/O sensor chip supports reading the fan speeds (assuming you have at least a 3-wire cable to the fan) via the motherboard fan headers. I use an excellent
non-free product named Aida64 which is a wonderful highly configurable hardware monitoring product which has many nice user-interface display options, although the following is mine:
There are other similar products out there both free and non-free, but much of what you'll be able to see comes from your motherboard's capabilities and what the I/O sensor chip allows you to see. For sure, you'll know when you look inside the BIOS's hardware monitoring outputs and see what they show you there.