How noticeable should the seam be on a quartz counter install?
I wouldn't sign the form if you were told that you wouldn't be able to notice the seams.  As Jack says, they are in horrible locations.  But the #1 thing I am thinking about is I have had granite and quartz installed on at least 30-40 kitchens, with at least 15-20 Ls.  You have two sheets, not 3 and your seam is in the corner.  If they are using smaller pieces they could easily be using leftovers from other jobs, maybe really costing them nothing more than labor.  I would have never allowed them to install 3 slabs there.  That is the biggest mistake.  

I have had many many companies propose something like this, a couple with no-see guarantees.  I took two of them up on it in the beginning and they both looked like yours.  Got the first one at 1/3 cost and told them to take the second one out.  Never did I go for the three slabs.

Now the other issue is the install itself.  To me - and I am just seeing a picture - it looks like the slabs aren't exactly level with each other.  They may be off by 1/128th of an inch but it looks like they are off.  Then you have the epoxy filler which was not mixed with a matching sparkle nor was it sanded/buffed properly at the site.  I have paid a lot for high quality quartz and don't have pictures but the seams are much smaller than yours and really you couldn't see them unless you put a spot light on them.  

The question is did you get what you paid for.  Did this cost you $500-800 and you are expecting better?  If you paid full/normal cost you probably just need them to come take the counters.  You obviously aren't happy now.  Let me tell you by the looks of your seams, they aren't going to look this good in 3-4 years.  So you are seeing them in their best state.