I find that about a month before lambing I check my ewes over and condition score them. any triplets and any ewes with low condition scores are seperated off and fed one concentrates. In the last 6 weeks of the pregnancy the lambs do their most intensive growing and this can be quite a strain on the ewes even if they were in good condition before hand.
I had a ewe go down with low calcium this year and we managed to save her but in previous years we have not been so lucky - ironically though she was a single in great condition!
The other thing to check for is magnesium deficiency. Any strain can trigger this. We never had an issue with this until we rented some extra land and made haylage from it. When the ewes were in we were feeding the haylage and a few went down with low mag. Turns out that area is bad for that - the farmer waited for me to find out myself! Now that I know - I have a mag bucket out for the ewes when they are in the shed. Fingers crossed we had no issues with it at all this year.
One last thing - any time I have a ewe or lamb under the weather I give it a shot of either multivit or intravit (Multi - does what it says on the tin - mutlivitamin and intravit is a cobalt, selenium and vitamin B booster). These tend to kick start the sheep to recovery and cost almost nothing.