"Opera" can be countable or uncountable, depending on what you are trying to say.
Uncountable: a style of music
Countable: a story set in a particular fashion; or, a building in which opera is performed, an opera house.
So you may be a fan of opera (uncountable). But if I asked you what your three favourite operas (countable) are, you could say, for example, Madame Butterfly, Aida and The Marriage of Figaro.
If you say you went to the opera for three days, you mean that over a period of three days, you repeatedly went to the opera house to watch and listen to some opera.
"I made a New Year's Resolution to be more culturally aware. So I decided to watch one opera every day. I went to the opera for three days, but I found it very boring and much too expensive. So the day after, I watched football."