If you decide you need an inline table-valued function for performance reasons, try this:
USE [W_SE-MFG]
GO
/****** Object: UserDefinedFunction [dbo].[QtyOnHandNoExcl] Script Date: 2017-10-12 09:57:22 ******/
SET ANSI_NULLS ON
GO
SET QUOTED_IDENTIFIER ON
GO
ALTER FUNCTION dbo.QtyOnHandNoExcl (
    @SC nvarchar(30),
    @Wh nvarchar(3),
    @CIE varchar(1)
)
RETURNS TABLE WITH SCHEMABINDING
RETURN
    SELECT CONVERT(decimal(8,3), SUM(X.QtyOnHand1)) AS RET
    FROM (
            SELECT 'K' AS CIE, QtyOnHand1
            FROM SysproSStar.dbo.InvMultBin AS p
            WHERE @CIE = 'K'
                AND p.StockCode = @SC
                AND p.Warehouse = @Wh
                AND NOT p.Note Like 'EXCL%'
            UNION ALL
            SELECT 'N', QtyOnHand1
            FROM SysproSStarNZ.dbo.InvMultBin AS p
            WHERE @CIE = 'N'
                AND p.StockCode = @SC
                AND p.Warehouse = @Wh
                AND NOT p.Note Like 'EXCL%'
            UNION ALL
            SELECT 'O', QtyOnHand1
            FROM SysproSStarAust.dbo.InvMultBin AS p
            WHERE @CIE = 'K'
                AND p.StockCode = @SC
                AND p.Warehouse = @Wh
                AND NOT p.Note Like 'EXCL%'
        ) AS X;
GO