user61874: 
For some reason, my 3.3v on my Arduino is fluctuating a lot, and I am not sure why.
The multimeter is measuring dc voltage and is connected between GND and 3.3v of an Arduino Mega.
The multimeter works fine. Connecting between 5v and GND gives 5V on multimeter.

user57281: Arduino output voltages may "fluctuate" a few mV but what you showing us in that gif is really unstable.
What you should be doing is:

Disconnect everything from Arduino except the power source
measure the voltage again;

Is it stable? the problem was because of pulling too much current that causing voltage drop and instability .
Is it unstable?

Try checking with another MM and/or change auto-range to manual.

Is it stable when measuring the voltage with another MM? something was wrong with your previous MM.
Still unstable?

Connect the Arduino to a 9V power adapter and disconnect the USB cable.

Measure the voltage again, if it's stable the problem was because of some error in the cable or USB power source.
Voltage is still unstable?

change the 3.3V voltage regulator.

It should fix the problem if not report back to do some more in-depth problem solving.

61874: <p><img src="https://i.imgur.com/r9wYwGP.gif" alt="Imgur"></p>

<p>For some reason, my 3.3v on my Arduino is fluctuating a lot, and I am not sure why.
The multimeter is measuring dc voltage and is connected between GND and 3.3v of an Arduino Mega.</p>

<p>The multimeter works fine. Connecting between 5v and GND gives 5V on multimeter.</p>
 57281: <p>Arduino output voltages may "fluctuate" a few mV but what you showing us in that gif is really unstable.</p>

<p>What you should be doing is:</p>

<ol>
<li>Disconnect everything from Arduino except the power source</li>
<li>measure the voltage again;</li>
</ol>

<p>Is it stable? the problem was because of pulling too much current that causing voltage drop and instability .</p>

<p>Is it unstable?</p>

<ol start="3">
<li>Try checking with another MM and/or change auto-range to manual.</li>
</ol>

<p>Is it stable when measuring the voltage with another MM? something was wrong with your previous MM.</p>

<p>Still unstable?</p>

<ol start="4">
<li>Connect the Arduino to a 9V power adapter and disconnect the USB cable.</li>
</ol>

<p>Measure the voltage again, if it's stable the problem was because of some error in the cable or USB power source.</p>

<p>Voltage is still unstable?</p>

<ol start="5">
<li>change the 3.3V voltage regulator.</li>
</ol>

<p>It should fix the problem if not report back to do some more in-depth problem solving.</p>
