Jeffrey,
ES futures are $50 per POINT, and the NQ futures are $20 per POINT.
Therefore, 6 ES = 6x ($50x S&P500) = 6x ($50x1043.75) = 6x $52,187.5 = $313,125
Or as I previously posted 6 ES = 3000 SPY = 3000 x 104.375 = $313,125
Yes, some brokers allow $2500 daytrading margin per contract. (Which significantly increases the leverage and is a double edge sword. I don't recommend novice futures traders to daytrade with such high leverage. You should learn as much as possible before trading with more than 4:1 leverage.)
Therefore, if you want to trade three contracts per trade, you must have at least 3x$2500 = $7,500.
(Keep in mind that you can trade 1 Dow emini futures contract with $1080 (initial margin), not $2,500 )
Fohat