Hamilton's Mercedes did not have the pace to keep up with his two rivals.
"There's nothing you can do," Hamilton said. "It's difficult when people pull away from you and there is nothing you can do about it.
"I feel happy we've extended our lead [in the constructors' championship] ahead of Ferrari, and also I'm up to third [in the drivers' ahead of Lotus's Kimi Raikkonen], so I have come and done what I planned to do.
“ I'm hoping we'll be more competitive from Singapore on”
Lewis HamiltonMercedes driver
"But it is difficult when he is pulling away at the rate sometimes he pulls away."
Vettel is 46 points ahead of Alonso, who is second in the championship, and is well on course for his fourth consecutive world title, a feat only Juan Manuel Fangio and Michael Schumacher have previously managed.
"But we have had some really strong races and we've been competing with him and we have finished ahead of him in the past so it doesn't mean we can't do that in these next races. But it is getting closer and closer to the point where we need to be finishing ahead every time to close that gap."
Hamilton said he believed his Mercedes team's form in Spa meant Vettel and Alonso would start favourites at the next race, the Italian Grand Prix on the high-speed Monza track on 8 September.
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