NVidia limits their graphics drivers to only 2 concurrent transcoding streams at a time for most non quadro graphic cards, it’s an artificial limit put by NVidia in their drivers which is why you 3rd simultaneous conversion fails when the driver tries to start it. The solution is to contact Nvidia or don’t limit your conversions to only Nvidia (set it to Any).
Some Quadro cards have unlimited concurrent transcodes (unrestricted). Here’s the table matrix showing the limits for different cards:
Here’s a discussion thread about it:
Here are two other links which discuss how to patch the limits:
More reading: