Here is the thing. 300MB is top theoretical speed. No-one outside a test lab will ever see that. Each wireless device you add to the network will chop that max speed in about 1/2. Overhead for wireless transmission will add another 10%. Distance will also affect this a lot, the further away you are the less speed you will get.
For 3 devices with any type of distance between them, 72 is just fine. 300mb/3 devices minus wireless overhead and distance. You are at your max speed. Don't forget the max hard drive throughput, your data transfer will go only as fast as your slowest hard drive.
The other poster is correct about internet speeds, but I'm guessing the 72MB you put as the speed is not your internet download speed, only a tiny percentage of people have that high of a speed to their house. And you will not get 300mb speeds between 2 devices anyway, closer to 200 with nothing else on the network and no distance issues.