+
: 足{た}す
-
: 引{ひ}く
/
: 割{わ}る
*
: 掛{か}ける
And you just say the terms normally in order. So your example of 3 * 4 = 12
would be 3かける4は12. Note that = becomes は, similar to how we use "is" in English. As @blutorange mentioned, you can use イコール to mean "equals," however in most situations you'll be good using は.
You learn these things quickly when listening to students recite their 九九{くく}.
For some bonus terminology, you can refer to exponents by using #乗{じょう}, so like 2の二乗は4 or 2の3乗は8. Similarly you can refer to roots with #乗根{じょうこん} (although as @jovanni points out it's normal to use 平方根{へいほうこん} for square roots).