I have a PiTFT touchscreen and which I have it onto Raspberry Pi 3, I want some other peripherals attached as well but the screen occupies all the raspberry pins. Is it possible to reduce the pins needed by the screen, or even use some other protocol such as DSI?
The PiTFT does not use all GPIO pins, the 40 pin connector is merely for physical stability and ease of use. Some PiTFTs even have another 40 pin GPIO connector in order to easily allow for more GPIO peripherals. You might want to get one of those.
Using DSI instead of the GPIO would be hard, as you would have to write your own drivers (and might need custom made driver boards as well). The Raspberry Pi Foundation allowed some insight on these issues when they introduced their 7" display.