"Three dog night" is an idiom for a freezing, bitterly cold night, originating from indigenous Australian or Siberian traditions where people slept with dogs (often dingoes) for warmth. The coldest nights required three dogs, creating a scale of temperature.
Temperature Scale: It is often used to describe, in increasing order of coldness, a "one dog," "two dog," or "three dog night".
Usage Examples
"It’s going to be a three dog night," the farmer said, looking at the freezing rain.
"Bring in extra firewood; it's a real three dog night out there."
- From online source with a bit editing
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March 20th is supposed to be the first day of spring, local schools are on their last day of spring break. But winter doesn't want to leave. It has been very chilly outside the past few days here.
Being a dog lover, I meet and pet them almost everyday. The day before yesterday when I petted the first two during my walk, I felt more warmth from them than usual.
Then suddenly, I thought about doing a dog APAD, so I pulled out my cell and found this idiom. "But, am I going to meet the 3rd dog?"
Sure enough, here he was, just before the end of my walk, wagging his tail and was very happy to be petted (see pic below). 
Hooray! I got three dogs for a "three dog night"!
Hope All are warm and cozy, wherever you are and whatever you do.
BTW, there is an American rock band named "Three dog night". Below is "Joy to the wrold" from their top hits, when/if you have time to listen.
