严格地说是八种
1:(3 + 3 ÷ 7) × 7
2:(3 + (3 ÷ 7)) × 7
3:(3 ÷ 7 + 3) × 7
4:((3 ÷ 7) + 3) × 7
5:7 × (3 + 3 ÷ 7)
6:7 × (3 + (3 ÷ 7))
7:7 × (3 ÷ 7 + 3)
8:7 × ((3 ÷ 7) + 3)