Congrats on the referral! There are a couple of things to consider.
#1, most likely this little boy has some delays due to being in an orphanage. Most of the time, you can count on 1 month delay for every 3 months of age. Extrapolate that out, and a 3 year old could very well be at the developmental level as a 2 year old. So, while your youngest child and this little boy are close in age chronologically, they are probably not close developmentally. This would make his adoption less disruptive of your birth order.
I have a friend with 8 & 10 year old boys. They adopted a 9 year old from Central Asia. Due to life in an orphanage, the 9 year old was more like a 6 year old maturity wise, developmentally, and socially. Even though he was in between their boys age wise, he really is in essence the youngest boy in their home.
#2, how is your 3 year old going to handle sharing attention with someone close to his age? Most 3 year olds can handle (to some extent) mom giving love and affection to a "baby" but it is harder to see them love and give attention to someone their own age. I think it is doable, but a 3 year old coming from an orphanage could have some attachment issues and you will need to be able to spend very quality time with him. Your 3 year old child now might have a hard time with that.
I hope your family can come to the best decision for your family and for this little boy!
Karla