I have a 48L tank that has been set up for 3 weeks now. I have 3 small common goldfish, that I placed into the tank one by one (I bought one fish 3 weeks ago, after the pet shop tested my water, another fish 2 weeks ago, and the third this Saturday: 30 June)

The water parameters as of today are:
Ammonia 0.25ppm
Nitrite 0.25ppm
Nitrate 20ppm
pH 7.5
Temperature 22-23 degrees Celsius

I perform 20% water changes every week, and I am using "Love Fish" water conditioner. I also add Interpet's Filter Start every month (I also followed the initial guidance of seven doses every two days).
I have the PF1 Interpet filter and I have a 6 inch air stone for extra aeration.

Okay, so I may be slightly paranoid considering that I am a new fish keeper, but I make sure that I observe my fish every day to make sure that they are healthy.
I'll "introduce" you to my fish (oldest first):
Oscar - small common goldfish - 1/2 pale orange, 1/2 gold - 3 weeks
Tango - small common goldfish - 1/2 dark orange, 1/2 silver - 2 weeks
[Unnamed fish] given a temporary name of Terry - small common goldfish - yellow - 2 days

I bought Terry two days ago, on Saturday 30 June and he is a healthy active fish, if not a bit shy. I was watching him all day Sunday and he was settling in well. This morning, Monday, I noticed that his cute little circular mouth has changed shape and it will not revert back to normal.
It seems as though the sides of his mouth have been 'pushed' in from the sides so that when his mouth is open, the orifice is an hourglass shape.
When it came to feeding this morning (I only feed the fish in the morning, pellets), seeing as Terry is shy, I observed him to ensure that he managed to eat a pellet. Oscar and Tango are quite cheeky and even though I only put one pellet per fish into the tank, one of them manages to snag Terry's. So I made sure one landed on the gravel for Terry to get at whilst Oscar and Tango are playing.
When it came to eating the pellet Terry found, he seemed to struggle to get it into his mouth, and when he (not sure if it's the right terminology) yawns, his mouth is still in the shape of an hourglass.
I have only noticed it this morning, but I'm really worried and I have a feeling that this problem is something that cannot be rectified, resulting in Terry having breathing problems and issues when feeding. Would it be best to take him back to the pet store and change him, or will he be fine? Has he injured himself, or is it a health condition?

Oscar and Tango are behaving as they usually do, swimming up to the front of the tank whenever someone enters the room and continuing their grazing when they clock on that they're not being fed. Terry's behaviour is the same as displayed in the tank at the pet store, thus he isn't any different apart from his mouth shape.

I have bought all three fish and all of my equipment from Pets at Home, and I have observed the tanks that the fish are kept in; they are all healthy looking fish.

Thanks so much for any help that anyone can give. I know I've written an essay, but I felt it best to give as much info as I could.

Common goldfish should be kept in tanks no smaller than 180 liters (That's for ONE of them.) If you want to keep goldfish, buy yourself a nice 150-200 liter tank and exchange these for three fancies instead. If you know anyone with a pond, you should probably give your goldfish away to them and keep the current tank as a quarantine tank only. It's far too small for most fish to be kept in it safely (even small tropicals).

I agree with Minde, the ammonia is a problem in your tank likely caused by the huge bio load on your filter from these three fish. You need to change out at least 50-70% of the water until the ammonia goes away, which it most likely won't as long as you keep all these fish in that small space. It sounds like there might be something stuck in the mouth of that one goldfish of yours, try seeing if you can spot any pebble lodged in there.

Hi Yogz, welcome to the forums,

I put together a thread about housing single tailed goldfish, so please check it out and give it some consideration.
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As the others have said, you need a much bigger tank.
I have a 3 foot/90 litre tank with one common goldfish and this isn't big enough for her as she will need upgrading again.

I agree with the others that the fish may have something stuck in it's mouth, if you spot anything in there, you could try removing it with some blunt tweezers or something similar that's clean.

Let us know how the fish does!

On small fishes the massaging might be really tricky... particularly if you don't know what you're doing or if you'd be too harsh you might do more damage than good I wouldn't quite recommend that. However, if the fish was suffocating on a rock, it would probably not be breathing by now.
When it happened to Samurai, she sank to the bottom and couldn't breathe at all. Just before I caught her to get it out she almost stopped moving, luckily it came out with a delicate push from behind the gills, but it was pretty clear that something was lodged in the throat.