In FX3 chip, SPI, UART, and I2S interfaces are multiplexed on the serial peripheral port.UART lines can be routed either on GPIO-GPIO or on GPIO- GPIO pins of FX3. When you are selecting the LPP mode as UART_ONLY the UART pins will be routed to GPIO- GPIO as per Table 7 of FX3 datasheet.
In case of CX3, the routing of UART pins are fixed, not multiplexed. In this case you have to change the LPP mode to CY_U3P_IO_MATRIX_LPP_DEFAULT instead of CY_U3P_IO_MATRIX_LPP_UART_ONLY. After making this change you can rebuild the firmware. The USB UART example will work with CX3.
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