How can I connect a third monitor to my MacBook Pro 13"?
Check this question out, looks like a similar question to yours.

How many monitors can the new retina MacBook Pro handle and at which pixels resolutions?  

Also, more specifically this:

How many monitors can the 13&quot; retina MacBook Pro output to?

Seems you need Thunderbolt and HDMI to do it. And if you're talking about the latest MBP 13" then you may be out of luck as that only has a single Thunderbolt port.

If you are on the new model you can't get more than one external display going without daisy-chaining through Thunderbolt which will cost you a bit unless you already have compatible monitors and hardware. DVI/HDMI/VGA won't cut it.

THOUGH...

If you wanted to go through USB apparently this is also an option:

https://gigaom.com/2010/10/07/quick-tip-add-more-external-displays-to-your-mac/

Buy a USB to DVI adapter, install DisplayLink drivers, and good luck