I am thinking of buying the Canon EOS Rebel XS SLR camera, while I have the "old" Canon film camera (AE1) with 3 camera lens: 50mm, medium zoom len and telephoto len.
Can I use these three old film camera lens with the new digital camera and, if so, what camera or len features I would lose, if any?
As you've heard, unforunately your FD-mount lenses won't work properly on the current EF/EF-s mount Canon camera. Canon made the change to the EF mount back in 1987 and it caused quite a bit of frustration for Canon users at that time. I know several people who moved to Nikon specifically out of anger at Canon's decision to abandon the FD-mount.
There are a few adapters to use FD lens on EF-mount bodies but there are some problems with this proposition. One problem is the fact the EF-mount is specifically an all electric mount so all actuation of the aperture and focus motor occur in the lens. This means the Rebel XS body has no way to control the aperture of the FD lens and it will have to be shot "wide-open." The second problem is the distance from the mounting flange to the sensor. The FD mount was only 42mm offset from the film plane where as the EF-mount is 44mm off the film/sensor plane. This means an FD lens would require corrective optics in the adapter to focus to infinity. At best, you could use the old lenses for macro work since the adapter would act like a lens extension.
For what it's worth, Canon's EF and EF-s lenses are worlds better than the FD lenses in many cases. The EF 50mm f1.8 lens can be had for less than $100 which makes the cost comparable to that of an FD to EF adapter to use your old FD-mount 50mm lens. The Rebel XS is often sold with 18-55 and 55-250mm kit lenses that are image stabilized and worlds better than your manual lenses. The point lurking here is that it may not be not worth your time to try to convert those old lenses.