If it would work for you, my preference, with a mother who seems to be in good condition and loving all three lambs, is to top up all three - or as many as will take some - in the field, plus give mum some cake to augment her nutrition.
Major advantages : you don't have to feed more than twice a day, all lambs have other lambs' company and mum's antibodies and protection, reintegration into the flock is a non-issue, lambs reared on mum with a top-up do as well as any other lamb reared on a ewe.
But crucially, two benefits over not topping up at all : 1, you have at least one lamb in the family established on a bottle, so you can always augment what the mother has and the lambs never become so demanding she gets tipped into mastitis, which can happen all too easily a few weeks in with a ewe who's doing it all on her own, and 2, all lambs are satisfied with what they're getting and so you don't get one lamb starting to pinch off other ewes because it's always hungry.
To get topping up established, you may have to bring the family into a pen twice a day and distract Mum with food (and it's good to get a bit of cake into her anyway) while you try each lamb with a bottle. I like to work on the basis that if I can get one lamb's ration into the family, then mum only needs to produce a twins' ration, so all should be well, and I've found it works well.
Over a few days, the lambs who like the bottle (which may be one, two or all three) learn to run to you for their bottle. And the ewe usually accepts the arrangement because she learns that you aren't stealing any of her lambs, just helping.
Over time, my experience is it usually evolves into one lamb never takes any bottle, one always takes a bit and gradually one needs most of a full ration, may even stop taking any milk off mum at around 3 weeks / one month, when the other lambs are taking all she can produce.