My 3 months old baby can now understand and do what I tell him to do. Is that maybe one of the signs that he is a smart one?
Your baby's reaction is normal.

It's expected that by 3 months babies should be able to interact or smile with others.

You should read this;
A language disorder may be spoken and/or written (reading and writing). It may also be receptive (understanding) and/or expressive (talking, reading, writing, or signing).
*Doesn't smile or interact with others (birth–3 months)
*Doesn't babble (4–7 months)
*Makes few sounds (7–12 months)
*Does not use gestures (e.g., waving, pointing) (7–12 months)
*Doesn't understand what others say (7 words (2 years)
* Has trouble playing and talking with other children (2–3 years)
*Has problems with early reading and writing skills—for example, may not show an interest in books or drawing (2½–3 years)