If you can get the C5 seats for cheap then go for it. They are fantastic in both looks and comfort.
As far as bolt in, not quite.
There are two ways I have seen this swap done.
First, remove the stock tracks from yous C3 seats, and customize them so they bolt onto the bottoms of the C5 seats, in which case the seats will bolt right in.
Second, depending on how tall you are and if you're comfortable losing adjustability...you can remove the C5 tracks and both the new seats right to the floor, making a few new holes to line up with the holes in the seats.
Obviously, one leaves you with adjustability but I have heard C5 seats sit higher and you might lose some head room.
The second option takes away adjustability and gains you a few inches of head room...which to taller folk, is a precious commodity.
I assume they fit dimensionally, again, you'll have to either drill new holes for the C5 tracks to fit in the floor, or you'll have to customize the C5 tracks with new bolting locations so they match up with the stock mounting location holes.