英语“3*5=15”怎么表达？

你的说法是正确的。
除此以外，还可这样表达：
1.Three times five is fifteen.
2.Three multiplied by five is fifteen.
以第一种说法为佳。

The product of 3 and 5 is 15.三和五的乘积是十五。
3 multiplied by 5 is 15. 3*5=15
（口语中一般略去multiplied）

3 is multiplied by 5 Equivalent to 15

Multipliedthree by five we get fifteen

Multiply 3 by 5,you get 15.

3 multiplied by 5 makes 15.