Both 1 and 2 are correct. #2 is used when referring to more than one painting.
In 3, the word "So" is modifying "many" -- telling to what degree. It's simply used for emphasis: Not just a few of us changed; MANY of us changed!
1. It is not good for the painting.
2. It is not good for the paintings.
(Are both OK? Which one is frequently used when we mention many pictures in a museum.)
3. How was the first day of school?
It was good. So many of us changed.
(In the dialogue, what is the meaning of 'So'?)
2 answers
2. is correct, paintings indicate more than one.
3. so in that context means to a great degree, so many then means great many. The same meaning would have been conveyed by "A great many of us changed."
3. so in that context means to a great degree, so many then means great many. The same meaning would have been conveyed by "A great many of us changed."