Don't tube feed a three week old puppy. Wean it. Use a bowl or puppy pan. At three weeks it doesn't need to be tube fed. It should not be tube fed, unless it has some birth defect.
Tube feeding is not optimum because if it eats or drinks on its own, you can actually put too much inside the dog. So if you're tube feeding you will have to stop any other forms of intake. Because you put the right amount in every so many ours. To do this, you have to shove a tube down their mouth, and after a week they have a gag reflex, so it gets much harder.
If the dog will not drink or eat out of a pan, then warm the food slightly, or give something different. But a healthy puppy will not starve itself to death.
I do not know if it is even possible to tube a puppy that is 3 weeks old.
That is a big puppy anyway, sounds healthy, it should wean just fine.
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