Welcome to the guild!

to answer the question, you do only need to attack them once , just attack the faction, you really don't need to attack them 3 turn if that was in your tribute deal.
The deal was to attack, once will do the job

Actually you really don't have to attack, I mean you will get your money.
The only thing that could happen when you do nothing , is that you get the '' I don't trust you to attack them '' message more often.
the faction(s) will trust you less, don't know that for sure though , for more info on the trusting factors of diplomacy read the diplomatic guide , stickied here in the colosseum