Whenever I download Raspbian Jessie 8 and install it, it automatically works with 7inch display and a full up monitor through ribbon cable and HDMI cable respectively. However, when I plug in the 3.5PiTFT or 2.8PiTFT display to the GPIO pins, I get nothing on the screen. I suppose I need to do something to the configuration file, but I am not sure what I need to change or add. Any help will be greatly appreciated. Thanks.
Based on your comments, it seems that there are specific GPIO pins that aren't working.
This answer and it's answers seems to address the issue, namely that one of the serial ports is being used by the Bluetooth of the Pi 3 (also applies to Pi 0)
The BCM2837 on the Raspberry Pi3 , Pi3B+, PiZeroW has 2 UARTs (as did its predecessors), however to support the Bluetooth functionality the fully featured PL011 UART was moved from the header pins to the Bluetooth chip and the mini UART made available on header pins 8 & 10.
You'll probably need to make some configurations to enable and configure the serial port, again per the answer above.