MongoDB presplitting chunks for compound shard key
Just a slight issue with how you are passing the $minKey values in, try this instead:

db.adminCommand( { split : "mydb.mycollection" , middle : { "region" : "region1", "foo" : MinKey , "bar" : MinKey } } );
db.adminCommand( { split : "mydb.mycollection" , middle : { "region" : "region2", "foo" : MinKey , "bar" : MinKey } } );


This got me the following layout:

sh.status()
--- Sharding Status --- 
  sharding version: {
    "_id" : 1,
    "version" : 4,
    "minCompatibleVersion" : 4,
    "currentVersion" : 5,
    "clusterId" : ObjectId("53a2cd9d98b4ace818666544")
}
  shards:
    {  "_id" : "shard0000",  "host" : "localhost:30000" }
    {  "_id" : "shard0001",  "host" : "localhost:30001" }
    {  "_id" : "shard0002",  "host" : "localhost:30002" }
  databases:
    {  "_id" : "admin",  "partitioned" : false,  "primary" : "config" }
    {  "_id" : "mydb",  "partitioned" : true,  "primary" : "shard0001" }
        mydb.mycollection
            shard key: { "region" : 1, "foo" : 1, "bar" : 1 }
            chunks:
                shard0000   1
                shard0001   2
            {
    "region" : { "$minKey" : 1 },
    "foo" : { "$minKey" : 1 },
    "bar" : { "$minKey" : 1 }
} --&gt;&gt; {
    "region" : "region1",
    "foo" : { "$minKey" : 1 },
    "bar" : { "$minKey" : 1 }
} on : shard0000 Timestamp(2, 0) 
            {
    "region" : "region1",
    "foo" : { "$minKey" : 1 },
    "bar" : { "$minKey" : 1 }
} --&gt;&gt; {
    "region" : "region2",
    "foo" : { "$minKey" : 1 },
    "bar" : { "$minKey" : 1 }
} on : shard0001 Timestamp(2, 2) 
            {
    "region" : "region2",
    "foo" : { "$minKey" : 1 },
    "bar" : { "$minKey" : 1 }
} --&gt;&gt; {
    "region" : { "$maxKey" : 1 },
    "foo" : { "$maxKey" : 1 },
    "bar" : { "$maxKey" : 1 }
} on : shard0001 Timestamp(2, 3) 


The use of the $minKey (MinKey) and $maxKey (MaxKey) values is a bit tough to tease out (they are rarely used except internally), but there is a decent, and illustrative example here in the docs.