Add a 4th table specifically for these values you want to be unique then link these keys from this table into the others using a one to many relationship.
For example you will have the unique table with an ID, AppName and ItemName to make up its 3 columns. Then have this table link to the others.
For how to do this here is a good example
Create a one to many relationship using SQL Server
EDIT: This is what I would do but considering your server needs you can change what is needed:
CREATE TABLE AllItems(
[id] [int] IDENTITY(1,1) NOT NULL,
[itemType] [int] NOT NULL,
[AppName] [nvarchar](20) NOT NULL,
[ItemName] [nvarchar](32) NOT NULL,
CONSTRAINT [pk_AllItems] PRIMARY KEY CLUSTERED ( [id] ASC )
) ON [PRIMARY]
CREATE TABLE Analog(
[itemId] [int] NOT NULL,
[Value] [float] NOT NULL
)
CREATE TABLE Discrete(
[itemId] [int] NOT NULL,
[Value] [bit] NOT NULL
)
CREATE TABLE Message(
[itemId] [bigint] NOT NULL,
[Value] [nvarchar](256) NOT NULL
)
ALTER TABLE [Analog] WITH CHECK
ADD CONSTRAINT [FK_Analog_AllItems] FOREIGN KEY([itemId])
REFERENCES [AllItems] ([id])
GO
ALTER TABLE [Analog] CHECK CONSTRAINT [FK_Analog_AllItems]
GO
ALTER TABLE [Discrete] WITH CHECK
ADD CONSTRAINT [FK_Discrete_AllItems] FOREIGN KEY([itemId])
REFERENCES [AllItems] ([id])
GO
ALTER TABLE [Discrete] CHECK CONSTRAINT [FK_Discrete_AllItems]
GO
ALTER TABLE [Message] WITH CHECK
ADD CONSTRAINT [FK_Message_AllItems] FOREIGN KEY([itemId])
REFERENCES [AllItems] ([id])
GO
ALTER TABLE [Message] CHECK CONSTRAINT [FK_Message_AllItems]
GO
From what I can tell your syntax is fine, I simply changed it to this way simply because I am more familiar with it but either should work.