In DS9 it pretty much said that while the Klingons were winning against the federation during their brief war at first that it was only a matter of time until they got their heads together and kicked the Klingon's asses, plus I believe it was said that the Klingons were contained.
In one episode, Worf outright says to his brother that the Empire has no chance of winning a war against the Federation. The whole theme of the Klingon arc through TNG and DS9 has been of an empire in decline, an empire dying from within due to corruption and rigid traditionalism. They were on par with the Federation during TOS but the strain of the cold war with the Feds eventually caused them to start crumbling and then the Praxis incident forced them to outright seek help from the Federation and eventually enter into an alliance with it. Their situation seems to have improved by the time of TNG but I don't think they can still be at the same strength as the Federation which has enjoyed near continuous expansion and progress for two centuries.
As for the Romulans, during TOS they always seemed like a power that was substantial but weaker than the top two powers (I think people often compare them to China, like they compare the Federation to the USA and the Klingons to the Soviets). Seeing how they entered a long period of isolation before TNG, I don't think they really expanded their power too much by the TNG times. They probably matched the Klingons, though.
I think the cloaking device is an important factor in my impression that the Federation is a stronger power than the Klingons and Romulans. I think the only way the Federation would be comfortable with not having cloak while its enemies and uneasy allies have it is if it knew it could nulify that advantage through sheer strength and numbers. And inversely, if the Romulans or the Klingons were as strong as the Federation while also having the cloak, they would have steamrolled the Federation a long time ago. All IMO, of course.