The problem is that neither
value: They contain a string with your time separated by colons: "12:35:56", or "23:00:00" - and the system will throw an exception if you try to convert that to a Double. Even if it didn't, adding two doubles will not give you what you need as adding 10 seconds to 55 seconds will not give you 1 minute and 5 seconds - it will give you 65 seconds.
To do this, you would need to parse the string into a Timespan and add those:
Dim s as String = aa.ToString("00:") & bb.ToString("00:") & cc.ToString("00")
Dim ts as TimeSpan = TimeSpan.Parse(s)
Dim result as TimeSpan = ts + ts