Q3 is an N-channel MOSFET (SSM3K15ACTC,L3F)
• Pin 1 Gate (G) on net “DescriptiveNet48” via series resistors R13/R14
• Pin 2 Source (S) tied to GND
• Pin 3 Drain (D) on “Net 50”

Net 50 fans out to R15, R16, R17, which are the current-limit resistors for the red, green, and blue LEDs of LED1.

With gate driven high by the MCU (through R13/R14), Q3 conducts, sinking LED currents to ground and turning LEDs on. When the gate is low or floating, Q3 is off, isolating the LEDs.

A pull-down (R13/R14) on the gate follows best practice: ensure Q3’s gate never floats, so the MOSFET reliably stays off when the MCU pin is tri-stated.