She's not. Infinity implies she can keep her abilities running for eternity, but it doesn't mean she has infinite magic or can have a spell remain active no matter what you do to it. Meliodas and Escanor are far stronger, but she's far trickier.
And isnt that pretty OP? Being pretty much the Archmage of the sins and having infinite duration magic effs taking place is no joke, yes it´s not the equivalent of infinite power output, which would be even higher hax, but Merlin is still more powerful, Strength wise Escanor and Meliodas could try to take her out yes, but she has more magic spell arsenal to showcase, and while it´s true Meliodas Full counter would counter a lot of what Merlin could throw at him it doesnt work on 100% of what Merlin throws at him, like curses or perfect Cube spell casting, Meliodas cant counter any of that.
The issue of a 1 on 1 against Merlin is that if her abilities have no ending duration, she has thousands of years of experience and can cast pretty much a high amount of spells, she will always have the upperhand, now of course it all depends on what Merlin has to offer as far as spells like enchants and curses go, she could very easily screw Meliodas and Escanor if she has spells that could work against their weaknesses.
For instance, if she has acess to Goddess spells Mel would be fucked, or if she has acess to spells that would restrict Escanor power in anyway shape or form, no matter how small it would still work quite well as a counter.
If for every spell she activates she can have it last a infinity with no proper drawbacks to her, she would literally be saving tons more of magic power than the rest of the other spellcasters, this is a great advantage, Mel and Escanor would definitely lose a dragged out fight with Merlin.