3 week old puppy doesn’t want to walk anymore?

So why are you so lacking in BASIC COMMON SENSE, you have not taken either the formerly ailing (now deceased) puppy or the currently and similarly unwell puppy, to the VET..... to have a diagnosis made?

It may be Swimmer Puppy Syndrome - which anybody who had done PROPER READING and RESEARCH before embarking on having a litter would have read about, as a post-whelping concern... and would have KNOWN to be on the LOOKOUT FOR. However, there are NO VETS on yahoo, so no one be certain, from your limited description.  

Home treatment may not be adequate (by this point) esp since the last puppy did not survive, and many do not.  Take the ENTIRE litter to the vet ASAP, but esp the SICK puppy, he is not long for this world if this is the issue, w/o veterinary help.  All 3 week old puppies, should be able to walk.  

This may possibly be your fault due to the TYPE of bedding/footing in the whelping box, but there is also has a genetic component, making ANY affected puppies POOR CANDIDATES for breeding & the parents of this affected litter => SHOULD NOT BE RE-BRED.  Congrats on being another an excellent example of a BYB =refusing to see a vet, with obviously sick puppies.

https://www.dogingtonpost.com/swimmer-puppy-syndro...

So, he has difficulty defecating and now is making no attempt to walk and breathing heavily? It's past time for a visit to the veterinarian. 

Ask your vet or breeding mentor in that case.. I refuse to help back yard breeders you see.

What is wrong with you? He is exhibiting the SAME SYMPTOMS as the pup that died, obviously NOT brought to the vet, just let it die, yeah, that's the way-- and now you come to YAHOO ANSWERS to solve another sick pup? You should not be breeding ANYTHING< EVEN YOURSELF!!!! You are too stupid for words and you are being horribly cruel to these living creatures! OMG!!!!!!!

you are a monster you know that right?  dogs wont stop eating..you let them eat until they are that full is animal cruelty you sick SOB

This is very common when people breed their pets & become BYBers (backyard breeders).  Normal pet owners know nothing about breeding dogs or raising puppies & what you don't know is killing off your puppies.  You are not equipped with the knowledge you need to do what you are doing.  Ignorance means you don't have the knowledge necessary to do what you are doing.

Both the mama & the papa should have been Vet check to see if they could breed together.  Mama should have had prenatal care during her pregnancy & the mama & all the puppies should have been to the Vet within 24 hours after birth.  The wellness check was to see that the mama is okay & doing well & each pup would have been examined for any problems, genetically & physically.

Not having a clue as to what is wrong with the pup, there is no way any of us can help you.  Now your only hope is a Vet.  BYBers are the low lives of dog breeding.

He's ill. Take him to a vet asap.

take it to the vet immediately.