A controller with only 2 available outputs is being used to control a 3 stage device.
Three stage device control where lower stages must remain active.
Hardware or space limitations may prevent there from being enough outputs to control a three stage device.
Two controller outputs in conjunction with a DPDT (dual pole, dual throw) relay can be configured to control a three stage device. This will allow you to activate stage 1, stages 1 and 2 or all three stages by activating either one or both of the controller outputs. An example of when this might be used is to control three stages of heat or a three stage fan.
A standard "ice cube" type relay can be used (IDEC RH2B or similar) and should be wired as shown in the diagram below.
The controller should be programmed to activate only output 1 for stage 1, activate only output 2 for stages 1 and 2 and finally to activate both outputs for stages 1, 2 and 3.
Note that certain application specific controllers may not allow the outputs to be configured in the way described above and thus cannot take advantage of this setup.