Send them home, showered with honor and glory, and let their families and neighbors take care of them. Teach the rest of the population to revere the maimed veterans. You don't have to spend your own resources on them if everyone on the home front is eager to bring them casseroles and fit them with prosthetic limbs. They won't resent you (as much) if they get to boast of their heroic exploits and are respected by the rest of society.
This day is call'd the feast of Crispian.
He that outlives this day, and comes safe home,
Will stand a tip-toe when this day is nam'd,
And rouse him at the name of Crispian.
He that shall live this day, and see old age,
Will yearly on the vigil feast his neighbours,
And say "To-morrow is Saint Crispian."
Then will he strip his sleeve and show his scars,
And say "These wounds I had on Crispin's day."
Old men forget; yet all shall be forgot,
But he'll remember, with advantages,
What feats he did that day.
This works best if the wounded vets are a small proportion of the overall population, so that everyone else can afford to support them, but it sounds like that's the case. Three platoons is about 150 men, and the smallest single-country population Wikipedia lists for the Middle Ages is Norway's at 0.2 million, or 200,000. If your wounded population increases by a factor of 10, that's still only 0.75% of Norway in the year 1000. (The fact that you say "no steam engine yet" makes me think that you're talking about the later Middle Ages, when Norway was still only at 0.3 million but the whole European population had increased from 5.4 million (1000 AD) to 11.5 million (1500 AD).)
If your population is still having problems absorbing the wounded vets, you can retrain them for jobs that don't require as many limbs, like pumping the foot-bellows in a forge if you have one arm, or weaving or carving if you have one leg. People who employ wounded veterans will be seen as doing something good and honorable.