Bring value based on criteria from multiple variable number of sheets

Good morning. I'm trying to do a lookup with a single criteria across multiple sheets. I have Data sheet where I need to bring Cu based on Lab.Nr. from multiple variable number of sheets.

Thank you!

Perhaps this will help.
The 2016 version of Excel comes standard with Get & Transform aka Power Query.
1. Produce Excel tables of the data on the 1, 2 and 3 sheets by selecting 1 cell in each range and pressing the Ctrl + t keys
2. Use Get & Transform to make connections to each of the tables.
The Power Query Advanced Editor code is:

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3. Use Get & Transform to produce an appended table of the three connections.
The Power Query Advanced Editor code is:

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Note that when new data is added to any of the three tables on the 1, 2 or 3 sheets then go to Query Tools > Query and select the Refresh.
4. The formula for column B on the Data sheet may be: =INDEX(Append1[CU],MATCH(A3,Append1[Lab. Nr.],0))
Let us know if you have any questions.