Add a plus sign to the number and it is considered to be 6 7 8 3 3 900

Two by two is one, making it 1 or -1 is fine.

For example, 2 and 3 are one, (2-3) is -1, 4 and 5 are one, (-4+5) = 1, and the sum is 0

Divide these numbers into two groups, the first group is preceded by the plus sign, the second group is preceded by the minus sign, let a be the sum of the first group of numbers, b is the sum of the second group of numbers, then there is a + b = 3 + 4 + 5 + 6 + 7 + 8 + 9 = 42, a - b = -10, can be solved a = 16, b = 26. Now you can choose the number that is sum to 16 from , 9 + 7 = 16, that is, 9 and 7 are preceded by a plus sign, and the rest of the numbers are preceded by a minus sign; 9+3+4=16, that is, fill in the plus sign, and fill in the minus sign for the rest. Wait a minute.

The sum of any three odd numbers will not equal an even number, so there is no solution to this problem.

They are all odd numbers, and the answer 30 is an even number, and it is impossible to add any three to equal 30. So it doesn't hold.

In decimal, it is impossible to add three odd numbers to an even number. If you want to have an answer, you can only do this: 6 + 11 + 13 = 30, 6 is 9 upside down.

All I can say is that there can be many answers, and in the conventional way, it is impossible to add three odd numbers to get an even number, so the idea must be extended to use other mathematical symbols. Many of the following scenarios can be combined, and the principle is to create even numbers or zeros.

Option 1: 3! +11+13=30 （3!=6)

Solution 2: log3 (9)+3 +1=30 (log3 (9)=2,))

Option 3: Option 4: 1'+15+15=30 (the reciprocal of any constant is 0).

Scheme 5: 3 +3+1'=30 (3 =27 1'=0).

Option 6: 3 +9-3! =30 (uses a negative number).

Option 7: 11+13+(6)=30 (inverting 9 to 6 is suspected of being opportunistic).

Option 8: 1 (day) + 1 (hour) + 5 (hour) = 30 (hour) (change unit).

Scheme 9: 15+15+1=30 (30 in decimal is 33 in decimal and 15 is 16 in decimal).

Summary. It needs to be added with 4 numbers.

Equal to 30, the three numbers are added together, and the numbers can be reused.

There is no way to add three numbers to 30 in this question.

You need to use 4 numbers to add 7+13+1+9=305+15+1+9=3011+15+1+3=30

Equal to 30, the three numbers are added together, and the numbers can be reused.

The process of calculating and splitting the fingers is as follows: the sum of any three numbers in this question is equal to 30, and there is no answer even if the heavy mountain collides with the compound to make the brigade only match. 5+15+1+9=303+7+5+15=30