Helping lambs survive and thrive

My neighbours ewe had triplets just over a week ago and two nearly died their first night. With extra feeds of colostrum and a few nights out of the weather they have survived. I am topping them up twice a day and have been watching them with mum as much as I can however I rarely see them feeding and when they try she either walks away or lies down. I am concerned they aren't getting enough from mum. I was thinking I would carry on with two top ups a day until I see some worrying signs. We are due another cold snap tonight and tomorrow which worries me too. Any advice welcome! Thanks in advance:)