The output ratio of workshop 1 and workshop 2 is 4:7, and that of workshop 1 and workshop 3 is 5:3. The output of the first workshop is unchanged, and the ratio should be good. Take the least common multiple of 4 and 5, which is 20. Therefore, the output ratio of 1, 2 and 3 workshops is 20: 35: 12.

Solution: Let 35X be the output of the second workshop and 12X be the output of the third workshop.

35X- 12X= 1380

The solution is X=60.

Output of the third workshop: 35X=2 100.

Answer: The output of the third workshop is 2 100.