Japanese question help?

Question:

I have two question:

First:

Shiken no mae ni
Before the exam

Why do we put "NO" in the middle of it,but no "NO" in this kind of sentence:

Sannen mae ni
Three years before

Neru mae ni
Before sleeping

Last does this sentence make sense(in the way its put):

Shiken no mae ni,PS3 o ju jikan shimashita.

If order of the sentence is not correct can you please show me the correct way.
Thanks

Answer:

Okay I have a go,
1)

First:
Why do we put "NO" in the middle of it,but no "NO" in this kind of sentence:
Firstly you have to find out each noun-phrase is like combined phrase or isolated phrase.
and if it looks like isolated word/phrase, you have to use 'のNo',
if it looks like combined word/phrase, you do not use 'のNo'.

Shiken no　mae、 ni
試験の前、に　　　　　＝This sounds like ; at the preceeding time of the exam
（しけんのまえ、に）
Before the exam
= Shiken mae、 Ni <= you are also able to say this. this is the same.
試験前、に
（しけんまえ、に）
Sannen-mae ni
三年前、に = 'three-years-before' 'Ni'
Three years before
**************************************… cannot say 'Sannen' no mae ni
Sannen no mae ni
=>sounds like Sannnensei no mae ni = In front of Year 3 pupils/students
Sannensei=Year 3 pupils/students

Neru-mae ni
寝る前、に
Before sleeping
*******=>>>> You just do not say Neru(Verb) + No combination.
Verb(original form) +mae, Ni = before ...ing.

e.g. Asobu-mae, ni = before playing

Your sentence about PS3 is totally correct.

Shiken no mae ni, PS3 o ju-jikan shimashita.
=Shiken-mae ni, PS3 o ju-jikan shimashita. This is the same.
the latter one is more like linking-noun. that is all.

If you are not sure, just ask me more.
Cheers,