To load 3 cores for 5 seconds:

seq 3 | xargs -P0 -n1 timeout 5 yes > /dev/null

This results in high kernel (sys) load from the many write() system calls.

If you prefer mostly userland cpu load:

seq 3 | xargs -P0 -n1 timeout 5 md5sum /dev/zero

If you just want the load to continue until you press Ctrl-C:

seq 3 | xargs -P0 -n1 md5sum /dev/zero