The three tornadoes' winds reached 200 miles per hour.
Explanation:
In order to make a noun possessive, you need to add an apostrophe (') and -s if the noun is singular: Jane - Jane's. If the noun is plural, you should just add the apostrophe: tornadoes - tornadoes'. There is no need to add an additional -s since the noun is already plural.
When we call a noun possessive, it means that something belongs to it. In the example above, the winds belong to the tornadoes.
