仍然是单数，因为纸paper 为不可数名词，可量词张piece 需要用复数。
例如
I grabbed three pieces of paper to takenotes on.
我抓过三张纸来记笔记。
The researchers then compared the artificially aged samples to the real deal: three pieces of paper crafted in 15th-century Europe.
然后研究者比较了人工老化的样本和真实的纸张
