Query multiple databases
If the serial is on the main table from EACH database, then you can just join the tables together using the fully qualified name:
CREATE VIEW vWhatever
AS
SELECT (whatever you want to select)
FROM [Database A].[schema - usually dbo].DSNA_tblMaiin AS a
JOIN [Database B].[dbo].DSNB_tblMaiin AS b ON a.Serialnumber = b.SerialNumber
JOIN [Database C].[dbo].DSNC_tblMaiin AS c ON a.SerialNumber = c.SerialNumber
WHERE (whatever you want to filter by)
GO
If the assets were in one of the tables, but not all three (and you didn't know which one), you could use a UNION within the view to accomplish it - it will essentially bring all three tables together, appending the fields that you specify (of the same datatype) until you have one large "virtual table" ex:
SELECT col1, col2, SerialNumber, etc.
FROM [Database A].[dbo].DSNA_tblMaiin
UNION ALL
FROM [Database B].[dbo].DSNB_tblMaiin
FROM [Database C].[dbo].DSNC_tblMaiin
Dot notation is exactly what you're looking at. And if you were extending the query to another server, it would just be [ServerName].[DatabaseName].[ObjectSchema].[ObjectName] instead of [DatabaseName].[ObjectSchema].[ObjectName]