How do I keep one of three of my indoor cats from overeating?

I have 3 neutered male cats. Food is left out for grazing during the day. The oldest one overeats and is now 15 pounds while the other 2 eat properly (11 and 13 pounds). Prior to free feeding they weighed 13, 13 and 10 pounds. But the vet suggested free feeding because the middle one was food aggressive and would eat so quickly he would vomit when fed only 2 times per day.