When I play this card do i need to have 8str to get 3 power or I need to win with more then 8 STR?
make on example
On the top I put wrong image sry.
You need to have attacked or defended with a total STR of 8 or more
You need to count at least 8 total STR and win. You don't need to win with an STR difference of 8.
For "perfect" clarity, compare the text on Make an Example to the effects that say you must win/lose a challenge by 4 or more total STR.