(2) Sell 3 contracts in March, buy 6 contracts in May and sell 3 contracts in July.
It can be seen that butterfly arbitrage is a combination of two intertemporal arbitrages.
In (1) is: bull market spread+bear market arbitrage in (2) is: bear market arbitrage+bull market spread.
The principle of butterfly arbitrage is that arbitrageurs think that the correlation between the futures contract price in the middle delivery month and the contract price in the two delivery months will be different.