2 to 3 days sounds short for the handover so don't be afraid to ask for more.
First get a working local environment with source control, ide, build and release steps all up on running locally.
Then try and get an impression of the code quality by going through it briefly. If it looks bad then you may not get so much useful information regarding the implementation from your predecessor.
However everything regarding deployments, db servers, backup strategy , registrations etc. should be checked. Also all licenses for libraries etc and also a list of the most common errors (if they have a bug tracker tool this may be useful)
Also you need to see how helpful your predecessor is as I've seen several styles of handover form ones where the person giving the handover was friendly but misleading to where they gave sarcastic answers to the questions asked of them in a form of a questionaire (which while funny wasn't professional) to just plain disinterested.