I found this defect on my brand new sp3 (bought today). It's minimal, but it's present, and I'm worried it can worsen.
Do you think I should ask for a replacement unit?
See how it goes, if there are no other problems Id knock wood and see if a dab of glue would resolve the slight separation of the antenna compartment cover.
That little separation could get worse depending on how your handling it in the long run. Do you plan on using a case/cover? If you're using it bare/naked, I would consider returning it. Otherwise it won't functionality but its your call and if you're nearby a store where your purchased it. There are many posts including the accessories section regarding cases/covers which would help so that little defect is not noticeable and doesn't gets worse.
It's normal... but it shouldn't be on a $1000+ machine. That should be lined up perfectly - you can already see in OP's pics how there's a slight chip in the plastic which wouldn't have occurred if it was flush. If you have a MS store near you OP, return it - you'll get a new SP3 on site - I returned one for similar reasons (the casing/glass was uneven all the way around the Surface, recessed at some points, glass overlapping at others). Process takes a couple minutes and you're out the door with a new SP3 - best to check the new one in store though and make sure it's free of issues.
If you can't get to a MS store though, not sure it's worth the hassle of an exchange...
My is like that and it hasn't been an issue. I don't travel with it much so I'm not sure if wear and tear would make a difference. It's also a tough call if you want to start playing the exchange game. Despite what people say $1000 devices are not all identical and some defects are worst than others. I would be embrassed to tell you the number of iPads I've been through in my life.