I read that an average roman soldier would drink about 100 gallons of wine a year combined with the fact that I read wine in the Roman Empire would have been 15-20% alcohol. This is well over a liter a day and when taking into account the apparent disparity in % between modern wine and then it's more like three bottles of 700ml wine per day on average, of modern wine.
When taking into account outliers some must have been drinking in excess of 3 litres per day without a single day break.
If this is true how did the military maintain itself as the most coherent military in Europe?
Not sure where you're getting that information, they did drink a lot of very weak alcohol and vinegar, but much less strong wine than you suggest.
Roman soldiers typically would have "sour wine" -- eg vinegar or wine substantially turning to vinegar-- and mix it with water. This vinegar was referred to as acetum (hence our chemical term "acetic acid"). This would have the salutary effect of potentially killing off some of the bacteria and parasites that might live in water vessels; the mix of acetum and fresh water was referred to as posca.
No (unfortified) wine will be %20 alcohol; fermentation stops at somewhere between %12-14. Modern yeasts can go a bit higher-- but increasing alcohol levels will poison the yeasts at %16 today, and likely the yeasts the Romans had would have been less tolerant. Getting to %20 and higher requires distilling . . . the Romans seem to have had some distilling after 200 CE, but only a little.
Roman soldiers did drink "vintage wine" -- eg what we mean by the term "wine"-- as well, but there's no evidence of them drinking nearly so much of it as you suggest, and we have evidence that at least some commanders did not permit it while on campaign
'Avidius Cassius forbade the soldiers when on expedition to carry anything except bacon, hard tack, and sour wine.'
Pescennius Niger gave orders that no one was to drink [vintage] wine on expedition, but that they should all be content with sour wine. He also forebade pastry-cooks to follow the expedition, and ordered the soldiers to be content with hard tack.'
They also drank beer where available; in Roman Britain for example.
One of the major problems for an army was keeping water clean -- dysentery will destroy an army quickly. Without access to clean running water, a commander might have preferred his soldier to drink a fermented beverage. Fresh water was, in many cases, a dangerous proposition.
See:
The Food Of The Roman Soldier. (1918). The British Medical Journal, 1(2998), 678-678.
Davies, R. (1971). The Roman Military Diet. Britannia,2, 122-142.
Arthur Patch McKinlay. “Temperate Romans.” The Classical Weekly, vol. 41, no. 10, 1948, pp. 146–149.