Great!
You probably know all this anyway , but just in case.
Choose a spare that is as similar to her own lamb as possible. (If they're all the same size, choose the feistiest.)
Keep the triplet mum out of earshot if you possibly can, so she and lambie can't call to each other.
When the single is born, do not give it to mum straight away.
Rub foster lamb all over in birth fluids. In particular, all over the top of its head and around its tailhead and anus. (Sorry, but those are the key spots.) I usually rub the new lamb and the foster anus-to-anus to make absolutely sure.
If the single is a first timer, you can probably get away with just giving her the foster lamb and then once she is licking it well, her own.
If the single is an experienced ewe, put your finger (washed and lubed) into the vagina and stimulate the top (dorsal side) of the vagina - to fool her she is having a second lamb
, then a few moments later give her the foster lamb. Let her lick and nuzzle that; do not give her her own lamb until she's nickered at the foster.
If you have
any concerns that she's not fooled, tie the foster lambs legs together (one front to opp side back) so it flops and falls like a newborn
.
Best of luck!