In your sentences, "in front of", "beside" and "behind" are prepositions. Prepositions are used before a noun, pronoun or a noun phrase to show their relationship (such as position) with other words in the sentence.
Therefore, your sentences should be:
I am behind (prep) you (pronoun).
The car is beside (prep) the three [boys] (noun phrase).
He is *in front of (prep) you (pronoun).
* "of" is part of the entire preposition "in front of".
I hope this helps.