Can I get to Freiburg in less than 3:15 hours from Flight arrival

No. It takes just over two hours for the fast trains to your destination.

Do you know that Freiburg hotels (sounds like you are in a Sharing Economy location, but there is a tradition of perfectly legal Zimmer rental in Germany ... ) don't offer the free KONUS transit card you can get elsewhere in the BF? I only mention this because it might make a financial difference if you have to change housing choices anyway.

I think not. The fast ICE trains leave once an hour to travel from the FRA Airport to Freiburg (Breisgau). The 15:52 train that you might catch will not get there until 17:59, hardly time to find the lodging. Unless your plane is very early, I can not see you catching the 14:52 departure, arriving at 16:59.

https://reiseauskunft.bahn.de//bin/query.exe/en

FRA Airport -> Freiburg (Breisgau).

which will arrive on 14:45

You mean is scheduled to arrive a 14:45. You can never know for sure that you will arrive on schedule, especially if you are flying from North America. I've been on hour late arriving in Germany from the US three times.

But, nevertheless, the scheduled arrival would not allow you to get to Freiburg in time. You can only make it if your flight arrives much ahead of schedule, and you can't count on that.

I assume that the person with the key is not going to meet you at the train station, that you will have to go somewhere to get the key.

There is a direct ICE connection scheduled to arrive at Freiburg Hbf at 17:59 (one minute before 6 pm). I think that is too late. It leaves FRA at 15:52, which gives you 1H07 to get off the plane and to the Fernbahnhof. In my experience that would be tight, but doable, particularly if you buy your ticket online in advance.

The next earlier connection, which arrives at Freiburg at 16:59 and might give you enough time in Freiburg, leaves FRA at 14:52. Seven minutes would not be enough time to catch the train. The flight would have to arrive really early to catch that train.