RA3 pin is also MCLR and VPP. RA3 can only be configured as an Input.
To enable use of RA3 (default its MCLR) you have to set following configuration bits, MCLRE and LVP.
#pragma config MCLRE = OFF // MCLR Pin Function Select bit (MCLR pin function is digital input, MCLR internally tied to VDD)
#pragma config LVP = OFF // Low-Voltage Programming Enable (High-voltage on MCLR/VPP must be used for programming)
