I would like to make inquiries about speech therapy. my son of three years is not yet forming sentences and engaging language as he should ?

Reasonable : By 3 a kid should form short sentences & be understood easily by strangers. Your doc should know a speech & language professional in your region. In the USA at age 3 yr the public school systems are required to seek out & offer services to such kids.

His pediatrician: Would be your best resource as he or she can examine in dynamics, make recommendations and further coordinate locally, get a speech therapist involved if needed.