I have a Mamba F405 Mini MK2 that I would like to use. According to the card it has three UARTs. I see TX/RX3 and TX/RX6, that's two of the UARTs, but what is the third? Is it the SBUS?

The quad I want to use it in will have a DJI Air Unit (want OSD), Crossfire RX, a GPS module, a buzzer and a servo. Buzzer has it's on connectors and I'll put the servo on the LED strip. So that leaves the three UARTs for Air Unit, RX and GPS. Each of these units require two cables (excluding power and GND). How should I go about utilizing the third UART for this?

SBUS is on the 3rd UART (UART1). It's an inverted RX connection so unless you are using an FrSky receiver or an ExpressLRS receiver in SBUS mode without telemetry that UART is useless. You can of course try soldering direct to the UART1 pins on the MCU to bypass the inverter and to access both the RX an TX connections of UART1, but that will require some expert soldering skills.

No, I don't want to solder directly on the pins of the MCU, too small for me.

Thanks for the info. I think I'll just use a different FC with more UARTs for this quad, I have one laying around.