I'm not sure either, but some lambs never take to the bottle so I assume they are always tube fed until you can get them on to creep feed.
I'm just hand rearing my first so someone with more experience will come along. Mine is only one day old so we are tube feeding in the hope it might still suckle.
Bearing in mind though that lambs naturally suckle little and often, I wouldn't think tube feeding once a day will be enough.
I think bottle fed lambs at 3 weeks will be on about 3 to 4 feeds a day, so I'd assume tube feeding should be the same?
And if it is weak I wouldn't think it was sensible to give a whole lot in one go.
J xxxxx