The best way to transport your bike somewhere is to ride it...but that's not always possible. The best option is to go for a bike rack that fits onto a tow bar, however, not all cars have a tow bar fitted and they are not the cheapest option.
Mounting bike racks on the roof of your car is a cheap and secure option, but if your car is tall, or you are short (like me!), it can be a bit of stretch to get your bikes on and off the car. You should also watch out for height restrictions at the entrance to car parks...
Probably the most popular way of carrying bikes on a car is with one that straps on the back of cars. This is certainly a cheap option but it can be a fiddly effort to get the straps tight and secure, and you must be sure you are not overloading your carrier. Three adult bikes will often exceed the carrier manufacturer's recommended maximum load weight.
If you can fit your bikes into the back of your car be careful not to lay them on their right hand side (where the derailleurs and cogs are). This is because you may knock your gears out of alignment. If your wheels have quick-release skewers it will help you greatly if you take the front wheel off the bike (if you have hydraulic brakes remember to put a wedge between the pads in case the lever gets squeezed by accident).
Choose a steel rack for cheapness, but an aluminium one will be lighter and last longer.
Mr Cycles recommends Thule, Hollywood and Avenir car racks.
You can find the Thule range here: THULE
You can find the Hollywood range here: Hollywood