The expression of "Three-Days Priest" is a literal translation of "Mikka Bozu" in Japanese.
"Mikka（三日）" means three days.
"Bozu（坊主）" means a monk.
"Mikka Bozu" is an expression for the person who can give up easily. This expression derived from the trainning for Buddhist priests.
The Buddhist trainning is usually known as strict. It starts with the service early in the morning, then cleaning the temple, chanting, studying, and again chanting. Besides, each meal is simple. So there are always some people who quit the trainning within three days. Because they usually quit within three days experience of being a priest, a quitter is called "Mikka Bozu." "Mikka" isn't necessary to be exactly three days, but indicates a short term.
When Japanese people refer to this word "Mikka Bozu", it has a negative meaning. Mikka Bozu is often criticized as a person who has no perseverance or a person who cannot stick to anything.
But I want to stand for the quitter. A quitter for one thing, means a starter for another thing. In order to start something new, you sometims need to quit something, too.
In the past, I had some dreams.....I repeated trying and quitting many things, many times. After much experiencing, I can tell you now that I am a priest here, because I was a Mikka Bozu in the past.