It's a bit of a haul from central Buenos Aires to EZE. The smaller airport at AEP is even further away so I would say it's a good hour from point to point (that's in good traffic). In a perfect situation where you arrived right at 3;30 pm that gives you approximatly 1.5 hours to get checked thru customs and have 'all your luggage' pre-screened. Unless you have status expect the processing to take at least 30 minutes. You then have perhaps an hour to get to the gate, if you're lucky that is!

Good luck, Oh! Looks like no time for some of the best duty free shops in the world at EZE.