I know the words in, to, by are prepositions.
But here :
(The words `in', `by', `to' are prepositions.)
In the above sentence I think `in', `by' `to' is nouns.
Am I right?
No, they're prepositions. The inverted commas just separate them from the text to show that they are examples and not grammatical components of the sentence. 'In' can be a noun- we talk of the 'ins and outs' of something, which are the details of its structure, organisation, procdedures, etc.