It's slightly ironic how the gurus function/wind up in Trigger:
Melchoir, Guru of Life, makes weapons that kill (Masamune, Rainbow, etc.)
Gaspar, Guru of Time, ends up in a place where time ceases to exist, or else functions very strangely.
Belthasar, Guru of Reason, loses his sanity in the ruined future.
I think that must be the most brought-up irony/quirk in the Chrono series. I've seen it mentioned no less than a dozen times.
Anyway...
That may be if one looks at it one way. But another way of looking at those terms is philisophical aproach. In that case, Melchior is Platonic, seeing things as being in the mould of 'universals', ideas which stand behind things and give them meaning (ie. a chair is a chair because there exists a universal archetype of a chair that gives meaning to the idea of chair). Life then could be seen as referring the the more mystical forces of the world, such as life and being behind things. Then Belthesar is Aristotilian, seeing truth as lying not behind things in some mystical realm, but rather in the thing itself. This is more in line with the progress of science (and, indeed, Belthesar seems very much the scientist, does he not?), especially since it was first Aristotle who put forward the theory of the four elements, used for so long. Finally, Gaspar looks not at the static being of things, but rather their interaction and reaction, drawing meaning therefrom - the passage of time, in other words, or perhaps fate. Essentially, if all three were to look at a stone, Melchior would wonder what mystical properties may lie in it, or perhaps what makes this stone a 'stone'; Belthesar would be more concerned classifying it and thinking what use may be made of its inherent physical properties; and Gaspar would wonder what circumstance brought the stone to be in their path, what caused it to be fashioned, and where its fate leads, and determines its identity therefrom. You see, the Three really are a perfect match, showing three aspects of the world: the mystic, the physical, and the transient. What glories they must have wrought working and understanding all together! Anyway, that's my own view on the matter, though I am certain that the irony of it all, as you have said, was in fact intentional in the game making.
Anyway, in so far as combat goes, and the might of each one... it is difficult to say, as each has their inherent strengths, and would victor in different fields of combat. Physically, I think all three, despite their age, would be quite strong; they are of Zeal, after all, and to my mind that is every bit as impressive as being, say, a Numenorean or such, and I think the magic burning within their veins would gift them surpassing strength of mind and body, so that even in their old age they would be keen and strong-limbed, for old men, that is.
Now, if the battle is grand, and calls for strategy and keen tactical thought, then it would be either Belthesar or Gaspar. Belthesar has the sharpest and most cunning mind of the Three, evident in his complex plan taking into account thousands of years and entire kingdoms. In that regard, he is likely a master tactician, and would victor in such a long-reaching battle. Gaspar could pose a challenge, however, with his knowledge of the intricacies of fate and the like. He could guess more of probabilities, if not complexities.
If the battle is smaller scale, say single combat, relying on strength, it would likely be Melchior who would victor: I consider him the youngest of the Three (with Gaspar the oldest), and he would likely be the strongest, bearing the mightiest array of weaponry and armoury, and be best with swords and weapons of such a sort. In terms of magic, however... here Belthesar would lose. Despite being of Zeal, I never saw him as much of a true sorcerer, but more apt to look to science, or rather mingle magic with science, as with the Epoch. So in battle, he would be at a loss in these means. In fact, he would be worst off in any form of small battle, rather being keen for the greater and grander reaching tactics of wars and the changing fortunes of kingdoms: a commander, not a fighter. But between Melchior and Gaspar... here I think it to be Gaspar coming to victor, simply for his command and knowledge over time.
Poll