Medieval soldiers most assuredly had no problems with ganging up on opponents in an actual fight. A very large amount of the combat that medieval soldiers would have participated in would have taken the form of skirmishes, raids, and other minor actions rather than large-scale pitched battles. Contrary to stereotypes, contemporary notions of honor did not prevent medieval warriors (whether knightly or not) from being quite tactically competent and performing clever maneuvers. Here is a description of a skirmish which took place between English and French troops in 1356:

The English...hearing the sound of horses' feet, turned round and saw it was the enemy. They immediately halted, to wait for the French, who advanced at a gallop, fully determined what to do, with their lances in their rests. The English, seeing them charge full speed, opened their ranks on each side and let them pass through, so that no more than five or six were unhorsed [out of 200]: they then closed their ranks and fell upon the rear of the French.

Very sneaky, those English! As you can see from the quote, mounted soldiers would generally be preferred for scouting, raiding, and harassment duties. Sometimes these mounted men would be men-at-arms, while others might be mounted infantrymen, who rode to battle but dismounted for the actual fight. In defensive situations, even the men-at-arms would commonly dismount in order to take up positions behind hedges or on a ridge where they could be protected from a cavalry charge. During the actual fighting, there were no rules. Soldiers could be ganged up on, flanked, surrounded, stabbed in the groin, knifed when they were lying injured, etc. The importance and prevalence of ransom is often cited as a limiting factor on the lethality of medieval combat, but it would be wrong to think (as some do) that ransom made fights less violent. Soldiers were frequently captured only when they were too wounded to keep fighting.