You could store all their PIDs in an array and kill those:
pids=()
make command1 &
pids+=($!)
make command2 &
pids+=($!)
make command3 &
pids+=($!)
kill ${pids[@]}
If you are running all of these commands in the same script, then AFAIK the PIDs will not be reused, and you can be sure that you're killing the correct processes.