PTSD is also a form of grief. I suffered from PTSD. One of the things that I was able to overcome is that I was able to talk about it.
Have you asked your son why does he feel this way?
See, if you can get him to talk about his concerns, even at three-years of age, he will have concerns, after all a three-year old is as honest as you can come and will speak the truth.
It takes a long time to overcome and conquer any traumatic experience and no doubt you have reached that stage.
Talking is the best way to get to the bottom of it all. There may be something in the room that reminds him of the incident of your loss.
Have you thought about redecorating the room, just be painting it in your three-year old's favourite colour?
Our heart is like an onion, and as we peel away those layers, we can remove those many hurts that are burined there. There may also be other things that your young son is dealing with.
Regards