Building a furnace with more than one thermocouple for accurate temperature measurement
For uniformity, you should STIR the air inside the furnace.
Measuring with 3 thermocouples does nothing to ensure uniformity.
I've worked with people who design/test silicon temperature sensors.
The oil baths are always STIRRED.
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Be wary of abruptly switching 36,000 watts. The energy stored in the power lines will cause ARCS at any contact points on relays, or high (destructive) junction voltages on FETS or bipolars or SCRs.