I looked at the various options a few years back, and went for the roof rack version as being a relatively cheap option. Having said that, I bought the Thule carrier, much as DibDib did, and it wasn't exactly bargain basement, but was still cheaper than fitting a tow bar and its wiring.
The problem for you, with the roof rack route, is that you'll probably need three carriers on top of the cost of the bars, whereas many tow bar carriers will take two or three bikes as standard. I'd also support going with the trough ones, rather than the hanging down ones. Those where the bike hangs, have many of the same issues as the ones which attach to the rear door, but of course those using a trough support for the wheels are more expensive.
If you're not tall (and like some others earlier, I'm relatively tall at 6'3") the roof approach is going to be more problematic, and will either involve the aid of a taller person, or various precarious options involving standing on things. With a heavy bike (and especially the central one), this could be difficult. Even for me, I didn't find loading the bike up to be trivial, it's quite an effort to raise a bike up that high.
I never liked the option of rear carrier hanging off of the boot, partly because it made opening the door inconvenient, but mostly because a lot of people talk about problems with them, which you rarely see for the other two type described.
I'd also reinforce what was said earlier about attaching things. Anything which can fall off probably will, so don't leave any bags on, remove lights, probably computers, and anything else that can fall off, even if held on by a supposedly secure fitting. The vibration from a road surface at 70mph, and air flow induced effects is likely to be a lot greater than on a bicycle doing 15mph, and harder to notice, unlike something which starts to rattle annoyingly as you cycle along. Retrieving an errant bicycle part from the fast lane of a motorway is also not a sensible option, I remember watching the police removing some debris from a motorway, on a fly on the wall documentary a few years ago, and they really weren't keen on doing it, but had to, to minimise the risk of it causing an accident. They ran from the hard shoulder to the central reservation grabbing the debris as they passed, and then ran back, on both occasions waiting and looking very carefully. Slip over, and you would be buggered. The police would likely stop you if they found you trying it (and possibly other consequent issues, like being arrested).
A few years ago, on a FNRttC, someone did neglect to notice the Max Headroom risk of a car park that they happily drove into without any bicycles on, and then drove out of after breakfast, destroying several carbon fibre framed bikes on the maximum height entrance bar.
