At age 3 we sat down with our son and explained how fires burn things and if there is ever a fire in our house what we should do. We talked about nighttime fires and even had a drill - dad turned the alarm on after our son was in his bed (for pretend) and he showed us how he could run out of the house.
We told our son that one night when he was sleeping we would have a drill. Every day he would wake up and say "the alarm didn't go off."
Unfortunately, as you may have heard, adult (loud beeping) fire alarms don't wake a sleeping child. We tried it and it is true! Our son slept through 2 mins of the loud alarm - located right outside his room! They now make fire alarms in which you can record your voice saying "Get up there is a fire" or whatever message you think is appropriate. I believe you have to get them online at this time.
Just talking about it and having a drill has made me feel a little better about it. And hopefully is setting the ground work for when he is a little older and can truly understand the ramifications and urgency of a fire.
Good luck.