What is the plural of word "a" ..can we. Say, we have spent a beautiful night with friends for three days.
There is no plural indefinite article in English. You simply avoid the article. Hence: we have spent beautiful nights. For further reference: Why is there no plural indefinite article?
First of all, although this doesn't help you, I would argue that while there is no exact grammatical plural of a, words like many and several serve a similar purpose.
I had a beautiful night. [singular]
I had several beautiful nights. [plural]
But you need a different type of construction—one that uses a specific number. I can think of a couple of possibilities.
We spent three beautiful nights with friends.
For three days in a row, we spent a beautiful night with friends.
I think the first is a more common way of expressing what you want. The second may sound a bit odd, but it's understandable and not wrong.