3 week old kitten low weight/gain

Hi I'm new to posting but have been reading threads since my bella was 6 weeks pregnant. Her 5 babies are 3 weeks and 2 days old now and all doing well apart from the runt. I have weighed them since day 2 and they have all been gaining a good amount each day until they were 2 weeks old thats when the runt stopped gaining. She gains a gram then loses one every day. Her brothers and sisters weigh
332g
359g
322g
298g
But she is hovering around the 185-189g mark. She seems strong enough and walks well, doing the same as the rest of them.
I have tried moving the bigger ones to give her a chance to get a good feed from mum and have been giving her some kmr each day but still no gain. Should I prepare for the worst? Could I mabe offer some gruel kmr+kitten food? Thanks in advance

I would supplement with that kind of weight gain and difference from her siblings. The biggest reason not necessarily being calories, but dehydration. When they get lethargic they will eat less, and then it's a vicious cycle. The stronger she is, the more she'll fight to eat with the others. However, I also find it troublesome that she kept up then dropped off. I would intervene just to make sure she is peeing/pooping normally. Have you been able to see if she is elongating properly? Testing her turgor response and the color of her urine will tell you if she is well hydrated. If all that checks out it's possible she is battling parasites or some kind of infection. Or hopefully, just that she just needs to eat more! The only way you'll know that is to supplement with some KMR and check her weight. It should improve over 24 hours. Make sure you over dilute the first couple of days so you don't overwhelm her.

Thank you, her poop and urine seemed fine until just now when I noticed orange cake batter consistency poop on her back legs and around her anus. She has had around 7ml of kmr watered down slightly since 7am this morning (its 12pm now) so maybe it's parasites. I think I'll have to take her to the vet

Yeah the vet is a good idea. I would take the whole litter and mom, though. Orange poop can be indicative of parasites and if you treat one, you have to treat all.