ALTER TABLE Table1
ADD
[Field1] nvarchar(4),
[Field2] nvarchar(2)

ALTER TABLE Table2
ADD CONSTRAINT Foo PRIMARY KEY ([ID])

ALTER TABLE Table3
ADD [Field1] nvarchar(50)

IF NOT EXISTS(select 1 from sys.columns where name='Field1' and object_name(object_id) = 'Table1')
  ALTER TABLE Table1 ADD [Field1] nvarchar(4);
  
IF NOT EXISTS(select 1 from sys.columns where name='Field2' and object_name(object_id) = 'Table1')
  ALTER TABLE Table1 ADD [Field2] nvarchar(2);
    
IF NOT EXISTS(select * from sys.objects where name='Foo' and type_desc = 'PRIMARY_KEY_CONSTRAINT' and object_name(parent_object_id) = 'Table2')
ALTER TABLE Table2 ADD CONSTRAINT Foo PRIMARY KEY ([ID])

IF NOT EXISTS(select 1 from sys.columns where name='Field1' and object_name(object_id) = 'Table3')
  ALTER TABLE Table1 ADD [Field1] nvarchar(50);