Koech clocked seven minutes 59.65 seconds, crossing the line more than 100 metres clear of his nearest rival, compatriot Welsey Kiprotich, at the Rome Golden Gala.
The biggest upset of the evening came in the men's 1,500 metres where Morocco's four times world champion Hicham El Guerrouj finished in eighth place.
It was only the fourth time in eight years that El Guerrouj has been beaten in the 1,500, two of those coming at the 1996 and 2000 Olympic Games. El Guerrouj set both his 1,500 and mile world records at the Rome Olympic stadium.
Bahrain's Rashid Ramiz finished first in a season's best of 3:30.25.
Men's world record holder Wilson Kipketer won the 800 metres against a top class field containing six of the nine fastest runners in the world this year.
The 33-year-old Dane clocked a season's best of one minute 43.88 seconds in his first competitive race of the year.
Slovenia's Jolanda Ceplak also set a season's best in the women's 800 metres with a time of 1:57.68.
Ghana's Aziz Zakari won the men's 100 metres in 10.10 seconds running into a slight headwind. World champion Kim Collins finished third.
South African high jumper Hestrie Cloete-Storbeck won an intriguing duel with Russia's Yelena Slesarenko, who leads the world rankings with 2.04 metres this season, after clearing 2.03 at her second attempt.
Cloete-Storbeck was one of five athletes to remain in the running for a share of the $1 million jackpot on offer to any athletes who win their event at all six Golden League meetings.
The other are Tonique Williams (women's 400 metres), Felix Sanchez (men's 400 metres hurdles), Virgilijus Alekna (discus) and Christian Olsson (triple jump).
