Could someone please tell me how auto-focus works on my Nikon D3300 when using my wireless remote (ML-L3). Does it still autofocus when i press the remote button?
I can't find an exact source for that, but from my experience, since the D3300 lacks the custom settings menu and the AF-S and AF-C priority selection that D5xxx, D7xxx and upper level models have and since the remote only has one button (no half-press), the camera will try to acquire focus for a short while after pressing the button and then it will take a picture (it will treat this button press as both half-press and full-press in succession). If it can't confirm the focus, it may fail taking a picture. To remedy this, you have two options:
Set the AE-L/AF-L button to AF-ON (if you can't find your way to do this, Google "D3300 back button AF"). This will decouple the AF acquisition from the shutter button and hence the remote button will only act as shutter release. Thus, if you focus in advance (if it's possible), the remote will only trigger the shutter and won't bother with AF.
[somewhat related to the previous one] Focus in advance using whatever method you find convenient and switch the lens to manual focus (using the little physical switch on the side of the lens). This way the camera won't try to confirm anything related to focus and will take a picture right away when pressing the button on the remote.