From your latest description it reads as a lamb that is starving ,when a ewe has triplets to rear they may all get enough milk to start then often 1 starts to get pushed out and slowly this gets worse so the lamb has to drink water and try to eat grass , leaves to fill its stomache but at 4-5 wks old its rumen is not developed so it cannnot get sufficient nutrition and goes lean ,slow to move ,rinkly skin ( dehydration ) (hard dry poo ) and develops a large " pot belly " . It is on a knife edge ,if you do nothing it MAY survive but will always be under developed with a pot belly , if you try to help it MAY survive , it is often very difficult to get a lamb of this age to take a bottle ,it needs lots of patience , you could try milk in a bowl or bucket which it MAY drink ,tubing again is very difficult for a lamb of this age and not a long term solution but will help keep it alive . When you say it sucks aggressively ,does it actually get much milk or has his siblings taken all ? . Good luck

. A ewe has to be very milky and on the best of grass ,hard feed to rear triplets well