diff --git a/cmd/camtool/sync.go b/cmd/camtool/sync.go
index ef2cb56d2..502f0a6aa 100644
--- a/cmd/camtool/sync.go
+++ b/cmd/camtool/sync.go
@@ -30,8 +30,9 @@ import (
 )
 
 type syncCmd struct {
-	src  string
-	dest string
+	src   string
+	dest  string
+	third string
 
 	loop      bool
 	verbose   bool
@@ -46,6 +47,7 @@ func init() {
 		cmd := new(syncCmd)
 		flags.StringVar(&cmd.src, "src", "", "Source blobserver is either a URL prefix (with optional path), a host[:port], or blank to use the Camlistore client config's default host.")
 		flags.StringVar(&cmd.dest, "dest", "", "Destination blobserver, or 'stdout' to just enumerate the --src blobs to stdout.")
+		flags.StringVar(&cmd.third, "thirdleg", "", "Copy blobs present in source but missing from destination to this 'third leg' blob store, instead of the destination.")
 
 		flags.BoolVar(&cmd.loop, "loop", false, "Create an associate a new permanode for the uploaded file or directory.")
 		flags.BoolVar(&cmd.verbose, "verbose", false, "Be verbose.")
@@ -98,16 +100,23 @@ func (c *syncCmd) RunCommand(args []string) error {
 
 	sc := client.New(src)
 	sc.SetupAuth()
+	sc.SetLogger(c.logger)
+
 	dc := client.New(c.dest)
 	dc.SetupAuth()
-
-	sc.SetLogger(c.logger)
 	dc.SetLogger(c.logger)
 
+	var tc *client.Client
+	if c.third != "" {
+		tc = client.New(c.third)
+		tc.SetupAuth()
+		tc.SetLogger(c.logger)
+	}
+
 	passNum := 0
 	for {
 		passNum++
-		stats, err := c.doPass(sc, dc)
+		stats, err := c.doPass(sc, dc, tc)
 		if c.verbose {
 			log.Printf("sync stats - pass: %d, blobs: %d, bytes %d\n", passNum, stats.BlobsCopied, stats.BytesCopied)
 		}
@@ -154,7 +163,7 @@ func (c *syncCmd) syncAll() error {
 		if c.verbose {
 			log.Printf("Now syncing: %v -> %v", sh.From, sh.To)
 		}
-		stats, err := c.doPass(from, to)
+		stats, err := c.doPass(from, to, nil)
 		if c.verbose {
 			log.Printf("sync stats, blobs: %d, bytes %d\n", stats.BlobsCopied, stats.BytesCopied)
 		}
@@ -180,7 +189,7 @@ func (c *syncCmd) discoClient() *client.Client {
 	return cl
 }
 
-func (c *syncCmd) doPass(sc, dc *client.Client) (stats SyncStats, retErr error) {
+func (c *syncCmd) doPass(sc, dc, tc *client.Client) (stats SyncStats, retErr error) {
 	srcBlobs := make(chan blobref.SizedBlobRef, 100)
 	destBlobs := make(chan blobref.SizedBlobRef, 100)
 	srcErr := make(chan error)
@@ -220,6 +229,27 @@ func (c *syncCmd) doPass(sc, dc *client.Client) (stats SyncStats, retErr error)
 	}
 	mismatches := []*blobref.BlobRef{}
 	go client.ListMissingDestinationBlobs(destNotHaveBlobs, sizeMismatch, readSrcBlobs, destBlobs)
+
+	// Handle three-legged mode if tc is provided.
+	checkThirdError := func() {} // default nop
+	syncBlobs := destNotHaveBlobs
+	uploadClient := dc
+	if tc != nil {
+		thirdBlobs := make(chan blobref.SizedBlobRef, 100)
+		thirdErr := make(chan error)
+		go func() {
+			thirdErr <- tc.SimpleEnumerateBlobs(thirdBlobs)
+		}()
+		checkThirdError = func() {
+			if err := <-thirdErr; err != nil {
+				retErr = fmt.Errorf("Enumerate error from third leg: %v", err)
+			}
+		}
+		thirdNeedBlobs := make(chan blobref.SizedBlobRef)
+		go client.ListMissingDestinationBlobs(thirdNeedBlobs, sizeMismatch, destNotHaveBlobs, thirdBlobs)
+		syncBlobs = thirdNeedBlobs
+		uploadClient = tc
+	}
 For:
 	for {
 		select {
@@ -228,7 +258,7 @@ For:
 			log.Printf("WARNING: blobref %v has differing sizes on source and est", br)
 			stats.ErrorCount++
 			mismatches = append(mismatches, br)
-		case sb, ok := <-destNotHaveBlobs:
+		case sb, ok := <-syncBlobs:
 			if !ok {
 				break For
 			}
@@ -247,7 +277,7 @@ For:
 				continue
 			}
 			uh := &client.UploadHandle{BlobRef: sb.BlobRef, Size: size, Contents: blobReader}
-			pr, err := dc.Upload(uh)
+			pr, err := uploadClient.Upload(uh)
 			if err != nil {
 				stats.ErrorCount++
 				log.Printf("Upload of %s to destination blobserver failed: %v", sb.BlobRef, err)
@@ -268,6 +298,7 @@ For:
 
 	checkSourceError()
 	checkDestError()
+	checkThirdError()
 	if retErr == nil && stats.ErrorCount > 0 {
 		retErr = fmt.Errorf("%d errors during sync", stats.ErrorCount)
 	}