The best and most natural option is:
✔ "I doubt the position in which to place the word 'yesterday' in the sentence." (Sentence 3)
This sentence correctly follows the structure: "the position in which to place...", which is a natural way to express uncertainty about placement.
What about the other two?
1. "I doubt the position in which the word 'yesterday' to be placed in the sentence." ❌ (Incorrect)
- "To be placed" is incorrect here. It should be "is placed" or "should be placed."
2. "I doubt the position where the word 'yesterday' is placed in the sentence." ✔ (Grammatically correct, but slightly unnatural)
- "Where" can refer to position, but "the position where" sounds less precise than "the position in which."
Tip:
When talking about placement or position, "in which" is often better than "where" in formal writing. Also, use "to place" instead of "to be placed" when referring to where something should go.