3. Without changing the order of the digits, place operation signs between them so that the result of these operations is 1

$$
12345678=1 .
$$
3. First, it is necessary to recall how one can obtain a unit - by multiplying a unit by a unit, dividing a unit by a unit, or finding the difference between two consecutive numbers. Based on this, the search for a solution should be conducted. Here is one of the solutions: $12: 3: 4 \times 56: 7: 8=1$.

Answer: $12: 3: 4 \times 56: 7: 8=1$.