R3-67 is an instruction in the SCI instruction set, which stands for Signed Compare Instruction. It is an 8-bit instruction used to compare two values in the processor's registers or memory locations and set the condition code flags accordingly. It takes two 8-bit input values and returns a 0 if the values are equal, a 1 if the first value is greater than the second, or a -1 if the first value is less than the second.