If a + or - appear and the current number is undefined then set the current number to 0

Brainscrabler memory consists of three stacks: A, B, and C. The "current stack" starts out at A, then is rotated to B, then to C, then back to A again

When performing + operation and the current number is undefined should I put 0 or 1 on stack?
Also description says All stacks have an original value of 0. Does that mean stacks are empty or do they include 0?
When the current stack is A and operator < appears should I switch to stack C or do nothing?