You could do this:
DECLARE @nums varchar(max)
DECLARE @comma char(1)
DECLARE @xml xml
SELECT @nums = '1,2,3,1,1,2,2,3,3,3,3,3,3,3,4,5,5,5'
SELECT @comma = ','
SELECT @xml = CONVERT(xml,'<r><n>' + REPLACE(@nums,@comma,'</n><n>') + '</n></r>')
SELECT DISTINCT [Value] = X.num.value('.','int')
FROM @xml.nodes('/r/n') X(num)
ORDER BY [Value]