Will RockShox Dart 3's fit onto a Genesis IO bike?

The key measurement you're looking for here is the steer tube size and threading type. On most modern bikes (this one included) that's 1 1/8" thread-less. Most older bikes are 1" and threaded. The fork should fit just fine. That said, if you're going to be spending the money on a new fork, save up longer and get an air sprung fork. They're lighter, have more tuning options, and a better bump response. In my opinion the only people who should be riding coil sprung forks are downhill riders, but that's because they require more travel and a higher maximum load than can be provided by an air spring. For XC however, air is the way to go.