In Spanish, adjectives can be placed before or after the nouns they modify, but the position can affect emphasis or description.
In this case, the adjective "perezoso" (which means "lazy") would typically go after the noun in standard usage. Thus, the correct option is:
3
So, the sentence would be structured as follows: "Diego (1) es (2) un (3) chico perezoso (4)."
This means that the best place to put the adjective "perezoso" is in position 3.