The device mentioned above will allow you to run multiple devices at the same time...
You could also just chain the 3 together using MIDI in, out and thru...
The only problem with this is that the way MIDI is designed, / You'll only get 16 total channels over all three devices...
For instance: On ONE port or one 'string' of ports put together, you have 16 channels to work with...
You would chain the three together and then set the MIDI channels on each device to be different...
Instrument 1 Could get channels 1 through 4 / Instrument 2 Could get channels 5 to 10 and the final instrument could be 11 - 16...
Only using one port connected together / this is what you'll have to do...
BUT!!!
If you get the interface above / Using a computer, Each out port you use gets it's own set of 16 channels...
So you could have 16 channels for each device....
Hope this helps clarify a little...