دفتر همکارم در طبقه سوم آن ساختمان است.

Yes, دفتر can mean both office and notebook/exercise book.

In this sentence, it means office, because the rest of the sentence talks about where it is located: در طبقه سوم آن ساختمان است = is on the third floor of that building.

So Persian دفتر is a context-dependent word:

• دفتر = office
• دفتر = notebook

همکارم is made of:

• همکار = colleague / coworker
• ـم = my

So:

• همکارم = my colleague

This ـم is a possessive ending attached directly to the noun. Persian does this very often:

• دوستم = my friend
• کتابم = my book
• همکارم = my colleague

So دفتر همکارم literally means the office of my colleague.

Persian usually does not use a separate word like English of in this kind of phrase. Instead, it uses a construction called ezafe.

So دفتر همکارم is understood as:

• daftar-e hamkâram
• literally: office-e my colleague
• natural English: my colleague’s office or the office of my colleague

The ezafe is usually pronounced but not written in normal spelling.

It is there in pronunciation, even though it is usually not written.

The sentence is pronounced roughly like:

daftar-e hamkâram dar tabaqe-ye sevvom-e ân sâkhtemân ast

The ezafe links words together in noun phrases:

• دفترِ همکارم = the office of my colleague
• طبقهٔ سوم = the third floor
• سومِ آن ساختمان = the third floor of that building

So although the written sentence has no short vowels, a learner should hear the links.

در often covers several English prepositions, especially in, at, and sometimes on, depending on context.

Here:

• در طبقه سوم = on the third floor

So even though English uses on, Persian uses در.

This is very common. You should not expect a one-to-one match between English and Persian prepositions.

• طبقه = floor / story
• سوم = third

So:

• طبقه سوم = third floor

This is the normal Persian order:

• noun first
• ordinal after it

Compare:

• روز اول = first day
• فصل دوم = second chapter/season
• طبقه سوم = third floor

Yes. Both are possible, but they are not equally common in all contexts.

• طبقه سوم = the usual, very natural way to say third floor
• سومین طبقه = also correct, but often feels a bit more explicit or formal depending on context

In everyday location statements, طبقه سوم is the more common choice.

• آن = that
• ساختمان = building

So:

• آن ساختمان = that building

Persian does not have a regular word exactly like English the. Definiteness is often understood from context, possession, or words like این/آن.

In this sentence:

• دفتر همکارم is already definite because it is possessed: my colleague’s office
• آن ساختمان is definite because آن points to a specific building: that building

Persian often puts information in this order:

• subject/topic
• place or other complements
• verb at the end

So this sentence is structured like:

• دفتر همکارم = my colleague’s office
• در طبقه سوم آن ساختمان = on the third floor of that building
• است = is

Literally, it is closer to:

My colleague’s office on the third floor of that building is.

That final placement of the verb is very normal in Persian.

است is the formal written form of is.

So the sentence ends with the copula:

• است = is

In written Persian, especially in standard style, this is very common.

In everyday speech, it is often reduced or replaced by a colloquial form such as ـه.

So a colloquial version might sound more like:

• دفتر همکارم در طبقه سوم آن ساختمونه

But the written sentence with است is completely standard.

Because آن directly modifies the noun that follows it.

So Persian normally says:

• آن ساختمان = that building
• این کتاب = this book

This is different from possessive or descriptive constructions, where ezafe is used:

• کتابِ دوستم = my friend’s book
• ساختمانِ دانشگاه = the university building / building of the university

So آن behaves like a demonstrative determiner, not like a noun that needs of.

A careful pronunciation would be approximately:

daftar-e hamkâram dar tabaqe-ye sevvom-e ân sâkhtemân ast

A few notes:

• دفتر sounds like daftar
• همکارم sounds like hamkâram
• طبقه sounds like tabaqe
• سوم sounds like sevvom
• ساختمان sounds like sâkhtemân

The written form hides short vowels, so hearing the sentence is important.

Yes, that is grammatically correct.

Compare:

• همکارم = my colleague
• همکارِ من = my colleague

So:

• دفتر همکارم
• دفترِ همکارِ من

Both work, but همکارم is usually more compact and natural in everyday Persian.

Persian often prefers attached possessive endings when possible:

• کتابم rather than کتابِ من
• دوستم rather than دوستِ من
• همکارم rather than همکارِ من

It can correspond to either English phrasing. Persian does not force the same distinction English sometimes makes.

So دفتر همکارم can be translated naturally as:

• my colleague’s office
• the office of my colleague

In normal use, the meaning is simply that the office belongs to or is associated with your colleague. The English wording you choose depends more on style than on a grammatical difference in Persian.