I think you would be ok with having a couple of lambs on a third of an acre. Were the last sheep that grazed it all up to date with worming etc? How much grass is there now, as grass won't really grow again until spring now anyway due to winter temps. Sheep are fairly even grazers, and if you got your lambs in March it would still be at least 6 weeks or so before they were weaned and relying on grass as the main source of food opposed to milk, so I think you would be ok.