Who can answer this Mathematics Query: 3 x (1+2+3+4+5+6...) and what is its single digit?

3 x 1 = 3 ..........[convert to one digit = 3]
3 x 2 = 6 ..........[convert to one digit  = 6]
3 x 3 = 9 ..........[convert to one digit  = 9]
3 x 4 = 12 ..........[convert to one digit  = 1+2 = 3]
3 x 5 = 15 ..........[convert to one digit = 1+5 = 6]
3 x 6 = 18 ..........[convert to one digit = 1+8 = 9]
3 x 7 = 21 ..........[convert to one digit  = 2+1 = 3]
3 x 8 = 24 ..........[convert to one digit = 2+4 = 6]
3 x 9 = 27 ..........[convert to one digit = 2+7 = 9]
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Question:

What is the result of 3 x (1+2+3+4+5+6...) ? and,

what is its single digit (3 or 6 or 9 or others)?