What is the equivalent thevenin resistance of a digital output port on an Arduino Uno R3
You can't directly, at least assuming there is nothing between the arduino output and the microcontroller output.

Microcontroller manufacturers don't spec output resistance usually, but they do specify the worst case voltage drop under some load.  For example, for a micro running at 5 V they spec might say the minimum guaranteed logic high output is 4.0 V at 20 mA sourced (I'm making up these numbers as examples only, you need to check the datasheet).  Assuming the output when driving high is a saturated FET that can be modeled well enough as a resistor, then this says it drops 1 V at 20 mA.  1V / 20mA = 50 &Omega;.  That's the worst case, so is the highest resistance.  The actual resistance can be anywhere from 0 to that value.

However, modeling a digital output as a Thevenin source is only roughly right.  You can get some first order useful information from it, but you certainly don't want to use that for any kind of detail.  If you find yourself wanting that, then it's a good clue you need to step back and think about the design differently.

It would help if you explained why you want to know the output resistance of the micro.