I own a 2009 Prius.

I also had the Bones rack, and you do not want to put 3 bikes on it. I did. The arms that rest against the glass come off the glass and hang there, with the rack only being held onto the car by the straps. If you look on the Saris website the Bones rack is not recommended for the Prius.

They sell sell other strap-style racks for the Prius though.

You can get a 1.25" hitch installed on the Prius at UHaul. From my research the UHaul hitch (I think it's made by Curt) had a good reputation for reliability, at the time another company called Coastal also made a hitch rack but it had a terrible reputation (I think they stopped making it). At the time I could not find a 2" hitch for the Prius, though since then I've seen them online that claim to be class 3, but...I'm a little dubious about how they could actually accomplish that.

The nicer thing about my hitch rack is that the bikes don't bump against the bumper on the car, and I can lift the hatch with the rack on, or even with the bikes on the rack (I have a Saris Cycle-On rack).

The drawbacks of a hitch rack are -
1. I tried 3 hitch racks and none of them hold to my car 100% steady like my strap style rack did.
2. They're more expensive than the strap style racks.
3. "Hold by the wheel" style racks - if you just put it on your car and leave it on it's fine. But they're more work to take on and off the car. Also, I have a garage, but before that I live in an apartment and I wanted the "hold by the wheel" style of rack - they are waaaay heavier and bulkier than the strap style racks, would not have wanted to take the rack on and off my car and back and forth between my apartment all the time. Whereas the strap style rack would just fit in the trunk. I think some might be better than others, but my Saris Cycle-On is heavy and huge.
4. You can get a 4-bike "hold by the frame" style rack for the Prius. But you cannot get a 4 bike "hold by the wheel" rack for the Prius because anything that carries 4 bikes in that style of rack requires a Class 3, 2" hitch. Note that this applies to almost all small and midsize cars as they're not rated to have a 2" class 3 hitch installed on them.
5. I bike in the winter so I leave my rack on the car in the winter, and the hitch racks are generally made out of steel and start to rust, and my "hold by the wheel" style of rack collects some of the road grime that's kicked up by the rear wheel. Whereas my strap style rack was higher up and made out of plastic and this was all a non-issue.

I've probably written way to much, lol. Basically you have 3 choices -
1. A different strap-style rack designed for hatchback geometry (there's plenty of choices out there)
2. A hitch rack
3. A roof rack

As for fuel economy, I think the other poster in confusing hitch racks with roof racks. Roof racks on the Prius (well - actually on most other cars to) have a reputation for reducing mpg by 2-3 with just the rack on there, and noticeably reducing it with a roof rack plus bikes. The bikes end up pointing right into the wind and create wind resistance.

A rack on the back of the car doesn't seem to affect fuel economy by any noticeable amount. I watched it when I got my hitch rack, and was surprised to find that just the rack with no bikes made no noticeable difference , and surprisingly I didn't even see a difference in fuel economy when I was driving with the back on it.

The only racks I know of that reduce fuel economy are the roof racks. (I mean I imagine driving with the bikes on the back must reduce fuel economy by something, but it wasn't enough for me to be able to tell while watching the fuel economy reading).