"mini ON-OFF 3pin SPDT" then it will be most likely the middle prong and one of the outer prongs depends on which way you throw the switch to "power on" use a volt meter to check which way round you want to place the switch (on the continuity setting) just read Roberts PDF
strange as in the PDF quoted SPST on page 25
Two mini-toggle switches (SPST ON-OFF) connected via a voltage divider network provide an option of overriding the power setting to each dew channel to maximum (100%) when activated. If dew is forming, a sudden boost to heat the telescope optics is available by using the override feature. Just remember to turn it off as it does not turn off by itself. The controller implements overriding the power to the dew straps and applying 100% to either channel. There are a number of ways to do this. The toggle switches (only for channel 1 and channel 2), when ON, overrides the power calculation and applies 100% power to the associated channel (this is referred to as Override Mode). When the toggle switch is OFF, power for that channel is set to 0%, and then on the next power calculation the correct power will be determined for that channel (this is referred to as Normal Mode). The override switches could be omitted if the intention is to use the dew controller via computer, as the Windows application provides this feature (as well as the ability to disable the override feature after a time elapsed period at 100% maximum of 2 minutes).