An exemplary gas supply system, includes a plurality of first input lines for supplying gas, a plurality of second input lines, a plurality of third input lines, a first mixing tank communicating with the second input lines, a second mixing tank communicating with the third input lines, and a plurality of three-way valves. Each three-way valve is communicated with a first input line, a second input line and a third input line such that gas in the first input lines can be selectively introduced into the first mixing tank or the second mixing tank.