小学三年级英语下册中常见的单词缩写形式包括：
1. I'm = I am
2. you're = you are
3. he's = he is
4. she's = she is
5. it's = it is
6. we're = we are
7. they're = they are
8. isn't = is not
9. aren't = are not
10. wasn't = was not
11. weren't = were not
12. don't = do not
13. doesn't = does not
14. didn't = did not
15. can't = cannot
16. won't = will not
17. wouldn't = would not
18. who's = who is
19. what's = what is
20. where's = where is
21. how's = how is
22. let's = let us
这些缩写形式有助于学生在口语和书面交流中更快速、更简洁地表达自己。建议学生在学习过程中多加练习，熟练掌握这些缩写形式。
