Hello, Welcome to BW forums
Congratulations on the arrival of your LO
My LO also slept through from the DF to morning when he was very young. I think some just do/can. Our health visitor didn't think it was a problem and weight gain was steady. For me, the WU time wasn't great, more like 5am and that was morning, but still I got a long stretch of sleep.
I always counted 3hrs from that first feed, I know he couldn't have gone any longer at that age. If at the end of the day the 3hrly feed didn't work out for BT I just fed at BT too, even if it was quite close to the previous feed.
It's always lovely to see a newbie here - hope you'll stick around.