declare @T table
(
ColumnA varchar(50)
)
insert into @T values
('A1:Data1 A2:Data2 A3:Data3'),
('A1:Data4 A2:Data5 A3:Data6')
select substring(ColumnA, Pos.A1+3, Pos.A2-Pos.A1-3) as A1,
substring(ColumnA, Pos.A2+3, Pos.A3-Pos.A2-3) as A2,
substring(ColumnA, Pos.A3+3, Pos.AX-Pos.A3-3) as A3
from @T as T
cross apply (select charindex('A1:', ColumnA) as A1,
charindex('A2:', ColumnA) as A2,
charindex('A3:', ColumnA) as A3,
len(ColumnA)+1 as AX) as Pos