sql query for select multiple columns in single column

Question

i have one single table and in that table there are three columns company1,company2,company3.

below is my table data :

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Company1 Company2 Company3

ABC

Test1 Test3 Test5

Test2 Test4 Test6

testing testing2

and i want to combine these columns into single column with serial number like below :

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SrNo CompanyName

1 ABC

2 Test1

3 Test2

4 testing

5 Test3

6 Test4

7 testing2

8 Test5

9 Test6