Not tested, but with Bash you should be able to do this
Code:
#!/bin/sh
# Concurrent downloading!
rtmpdump -r URL1 -o vid1.flv &
rtmpdump -r URL2 -o vid2.flv &
rtmpdump -r URL3 -o vid3.flv
# Sequential downloading!
rtmpdump -r URL1 -o vid1.flv
rtmpdump -r URL2 -o vid2.flv
rtmpdump -r URL3 -o vid3.flv