Studio One won't be able to natively open a Garageband project file (and, conversely Garageband won't be able to open a Studio One .song project file natively, either). This project file contains info like where the audio clips start/end, each clip's duration, what effects have been applied to which track(s), the fader/pan position of each track, etc.
What you can do is to export/render all of the individual track data in Garageband to individual .WAV files beginning from a start point of 0:00:00 (so they'll line up properly in a new song) and then import those into Studio One 3. Then, at that point you'd be re-building your song settings, mixing, etc..