If you don't want to use a third party tool, you can get pretty close to simultaneous execution by running ssh then executing the command then back grounding it. I have done something similar to this and works pretty well. If you can have a tiny delay between when the first command executes and the last one does, cause there will be one. So it would be something like this...
ssh user@server1 'command' &
ssh user@server2 'command' &
ssh user@server3 'command' &
Since you background the process, it won't wait until that particular ssh command is executed to move on to the next one. You'll have to play with it, but you can usually do things pretty quick this way. I have written multiple scripts doing similar things to this for quite a few nodes to run something more or less at the same time. Again will be a small delay between first and last, probably less than a second.
Edited by DeimosChaos, 26 April 2016 - 08:05 AM.