I've been reading a bit into 14th century England and have come by the three estates, those who work, those who pray and those who fight. When reading into those who fight, I understood that they were composed of the nobility, Lords, knights, etc. But I imagine that armies weren't composed solely of nobility, but also some common soldier or what have you, and the nobles perhaps lead them during battle? So where do these guys fit? I'm guessing those who work couldn't be drafted to become soldiers, then maybe these soldiers don't fit the three estates, like merchants, but then, what did they do in times of peace?
Thank you in advance!
The concept of the three estates extended beyond England to encompass most of western Christendom. But you need to bear in mind that the estates were a theoretical arrangement of society created by clergymen in the High Middle Ages and thus not a reflection of society as it actually was. The truth is the estates overlap to such a degree as to be almost completely porous. I encourage you to not get too hung up on it.
Both merchants and common soldiers, since you brought them up, were generally lumped with "those who worked," because that was where everyone who wasn't an aristocrat or a cleric ended up. As you've pointed out, the concept is shaky even on its surface. Is an Italian banker who lends vast sums of money to the king of England to be lumped in with a landless serf? Obviously their life conditions varied more than they were similar.
To go into slightly more detail on the issue of common soldiers: they were always, always part of medieval warfare. Knighthood and the concept of the military aristocracy only emerged in the 11th-12th centuries, and this in turn gave rise to the tripartite division of society. Now, as long as they fought under the authority of a lord, as a retainer or as a levied man, everything was muy bueno. But, beginning in the 12th century, bands of common soldiers under their own captains began to hire out to the great men of Europe. This caused no small amount of social consternation (among theologians, anyway; kings and lords were delighted to hire reliable troops who fought as long as the pay kept coming) because it violated the theory of the three estates.
By the 14th century, mercenaries and contracted troops had almost completely replaced the levy in the Anglo-Franco world, which had never been as pervasive as earlier historians once thought. Contracted troops were rather different from mercenaries. It worked something like this: a lord, knight, or accomplished man-at-arms would arrange with the king to supply a certain number of soldiers of a certain type with a minimum standard of equipment for an upcoming campaign. The king would agree to pay each soldier a certain amount for each day he spent on campaign, the amount varying with the status of the soldier; dismounted archers received less than mounted archers, who received less than men-at-arms (unknighted knights, basically), who received less than knights, who received less than lords. This is almost a sort of proto-national army.
As to social background and what they did in times of peace? We're not entirely sure. We do know that many "common" soldiers came from fairly well to do families; by the Hundred Years War some poor aristocrats fought as archers and some men who had begun as archers later became men-at-arms. Others came from the tradesmen and merchants of towns and cities, who raised their own militias. Still others came from the rural peasantry, probably mostly from the upper ranks of free peasants. Many of these men would go back to their trades or farms in peacetime. Those who were retainers of lords remained in their masters' service. But there were always idle soldiers around Europe. Some went abroad to fight as mercenaries. Others, finding themselves unemployed in a foreign country, became more or less brigands, extracting their wages from the countryside.
I've had to generalize tremendously, and I hope I've not left too much out. /u/MI13 may be able to help you with more specific information on 14th century England. If I've been unclear about anything, please don't hesitate to ask for clarification.
Sources:
France, John. Mercenaries and Paid Men: The Mercenary Identity in the Middle Ages.
Barker, Juliet. Agincourt: Henry V and the Battle that Made England.
Turnbull, Stephen. The Knight Triumphant: The High Middle Ages, 1314-1485.
Duby, Georges. The Chivalrous Society and Early Growth of the European Economy.
Strickland, Matthew. Anglo-Norman Warfare: Studies in Late Anglo-Saxon and Anglo-Norman Military Organization and Warfare
The don't fit in, because as is explained at length in the FAQ and by my own post here, modern historians no longer give weight to the idea of Feudalism or the three estates (or at least say that they don't). Instead, they see the former as a construct of the 17th century and the latter as an idealization of reality written into the record by clerical authors, so that when we look at the situation as it was, we find that nothing really fits, as you have done here.
If you give me a sense of what you've been reading, I can direct you towards more recent work.