Bunn pour omtic 3 burner

I'm going to presume that your brewer is a VPS series pour-over brewer. This problem can be caused by several things, so I'll list the most common ones.
1) The on/off switch for the lower warmer could be burnt out.This switch also operates the heating element.
2) The high limit thermostat quite offen blows on the older VPS brewers. this is a small unit held down to the top of the tank and has 2 wires voming to it.
3) Look for burnt-off wires. These models sometimes burn off wires on the top of the heating element.
4) If none of these work, then your problem will be your heating element itself.
Parts for these models are readily available from your local Bunn repair shop.