document.json

doc = {
  name: "Samuel Flores",
  exams: [
    {name: "Eletro-cardiograma", date: "2012-08-10"},
    {name: "Hemacias", date: "2012-08-09"},
    {name: "Tomografia", date: "2012-08-10"},
    {name: "Hemacias", date: "2012-06-10"},
    {name: "Ressonancia Magnetica", date: "2012-05-10"},
    {name: "Whatever", date: "2012-04-10"},
    {name: "Whatever", date: "2012-03-10"},
    {name: "Whatever", date: "2012-02-10"},
    {name: "Whatever", date: "2012-01-10"}
  ]
}
// db.people.insert(doc);

query.json

// o limit(3) limita a quantidade de documentos da coleção retornados (people) e não a quantidade de exams. acho que vai ter que rolar um reduce mesmo :p
// db.people.find({}, {'exams': 1}).sort({"exams.date": -1}).limit(3);

{
  "_id":ObjectId("503e385c41949d36180e0f92"),
  "exams":[
    { "name":"Eletro-cardiograma", "date":"2012-08-10" },
    { "name":"Hemacias", "date":"2012-08-09" },
    { "name":"Tomografia", "date":"2012-08-10"},
    { "name":"Hemacias", "date":"2012-06-10" },
    { "name":"Ressonancia Magnetica", "date":"2012-05-10" },
    { "name":"Whatever", "date":"2012-04-10" },
    { "name":"Whatever", "date":"2012-03-10" },
    { "name":"Whatever", "date":"2012-02-10" },
    { "name":"Whatever", "date":"2012-01-10" }
  ]
}

// ou db.people.find({}, {'exams.date': 1}).sort({"exams.date": -1}).limit(3);

{
  "_id":ObjectId("503e385c41949d36180e0f92"),
  "exams":[
    { "date":"2012-08-10" },
    { "date":"2012-08-09" },
    { "date":"2012-08-10"},
    { "date":"2012-06-10" },
    { "date":"2012-05-10" },
    { "date":"2012-04-10" },
    { "date":"2012-03-10" },
    { "date":"2012-02-10" },
    { "date":"2012-01-10" }
  ]
}

solution.js

db.people.aggregate( 
  { $project: { exams: 1 } }, 
  { $unwind: "$exams" }, 
  { $sort: { "exams.date" : -1 } }, 
  { $limit: 3 }
)

result = {
	"result" : [
		{
			"_id" : ObjectId("503e688d156562dc0cb61603"),
			"exams" : {
				"name" : "Eletro-cardiograma",
				"date" : "2012-08-10"
			}
		},
		{
			"_id" : ObjectId("503e688d156562dc0cb61603"),
			"exams" : {
				"name" : "Tomografia",
				"date" : "2012-08-10"
			}
		},
		{
			"_id" : ObjectId("503e688d156562dc0cb61603"),
			"exams" : {
				"name" : "Hemacias",
				"date" : "2012-08-09"
			}
		}
	],
	"ok" : 1
}