If we take the Endgame tech and put it aside, Voyager underwent significant upgrades over it's 7 years of trip and would likely be able to surpass numerous SF ships in virtually everything.
But let's put those aspects on the side-line as well.
If all 3 ships start off with same technological enhancements they'd receive during/after the Dominion War ... then the victory can go to either of them.
Let's face certain facts:
The Defiant went up against a heavily upgraded Excelsior class ship and fought it to a stand-still.
Anti-Borg technologies (which underwent enhancements before they were put into the Defiant ever since the Enterprise-D got them and were not really effective against 'Hu' Borg) would be incorporated in Galaxy and Intrepid classes as well.
They wouldn't have pulse phasers, but their standard phasers would likely be just as effective.
The Defiant despite it's agility cannot escape phaser beams, and it's firing arc is much more limited compared to other two ships (of which the Intrepid is rather agile itself).
But, I would surmise that if all 3 ships would be dividing their firepower between themselves ... well, like I said, it would likely end up in a stand-off or the victory would go to the better tactician.
Intrepid was also built for compat per on-screen reference and was mentioned in off-screen interviews to have identical weapons/shields compared to the Galaxy.
Being a smaller design with much less luxuries and creature comforts, the Intrepid can afford it, just as the Defiant can afford it's very powerful weapons in a small design because it completely eliminated creature comforts and luxuries (only dire basics).
This is why larger ships can meet their matches in smaller ships/designs and why I think the battle could go either way.
The Defiant wouldn't be exclusive in it's use of Quantum torpedoes.
The other two would likely have them as well, and I sincerely doubt the Intrepid can hold a maximum of 36 torpedoes given that Voyager was launched for a specific mission to retrieve the Maqui and therefore we cannot make assumptions she came fully loaded or supplied when nothing in the show supports that premise.
In either event ... if all 3 vessels would start off on equal footing with all of them using quantum torpedoes in battle ... well, it could get ugly.
The Galaxy might take a good pounding because it's less agile compared to the other two and couldn't really avoid too many torpedoes (as slow as they were portrayed to move) ... then again, the Defiant trups both vessels in agility, so both the Intrepid and Galaxy could get a good pounding on their shields with pulse phasers and quantums.
The Defiant would also be pounded a lot from direct phaser beams of the other two ships and it cannot divide it's fire as easily because it lacks full phaser array coverage as the other two.