I have a bunch of nikon lenses at home which my parents has bought with a firm-slr camera about 20 years ago…
So if i buy a canon 400D or nikon 50D now(or a s3is), will they be compatible or did lens design change in the last couple of decades? 
Each lens has a specific lens mount, so it would be compatible only with cameras using that particular mount. In this case, your Nikon lenses would use the ‘Nikon F mount’ and can probably be used with the Nikon 50D. The Nikon F mount has been around for the past couple of decades, so they should fit. However they did improve upon the F mount over time to support newer technologies (w.r.t metering/focussing) so there may be slight incompatibilities or you may not be able to use the most of your camera/lens combo. This should give a more detailed insight into the F mount : Nikon F-mount - Wikipedia, the free encyclopedia
The Canon uses another type of lens mount (EF/EF-S if i’m not mistaken), and the S3IS does not have a removable lens.
my parents has bought with a firm-slr camera about 20 years
You mean the FM SLR camera?
i meant film-slr’s
i guess ill have to find a d50 and check if the lenses can fit
They should fit there’s no doubt about that.. how well they interface with the camera is the issue 
If you have the time you could list down the bunch of lenses you have. Somebody with experience could probably tell you if they’re really good lenses worth salvaging as well as how compatible they are.
You’re in luck! Nikon and Pentax did not change their lens mount system in the late 80’s like everyone else did when Autofocus technology was introduced. So your lenses will work on a new body, however you may lose out on some features.
Also note that while the mount is the same, some older lenses (pre 1977) are Non-AI lenses and may damage the newer cameras. Why dont you post more info about the lenses here, like exact model number…
S3 IS is not an SLR, and does not have removable lenses.
all old nikon lenses can fit the latest digital SLRs but they were never made for digital so u wont get quality results.
The interface of optics have to be different or digital cameras, I have tested 11 lenses & found that buying new lenses was the only option to get the best out of the camera…later now I switched to canon.
Dont waste ur money bcoz the lenses can fit, but they wont get u good results.
Nikon D50 is out of production. If these lenses are autofocus, then you need to get a D80 to autofocus them. If they are manual, D40 is fine. The D40 can only autofocus with newer DX lenses meant for DSLRs.
will be able to post exact info when i go home in 2 weeks
on standby till then
heres a list of the stuff thats been stored away in deep freeze for a decade
What will be usable on the new cameras?
and any idae on how much the fg and f501 should sell for nowdays?
nikon fg
nikon f501
nikon e series 36-72 3.5
nikon 50 1.4
formula 5 52mm skylight1
makinon mag +1+2+4
cherry 58mm delta? pl
" 52
sigmo alpha 3.5-4.5 35-135 phi55
vivitar 2x macro teleconvertor
tamron sp 3.5 70-210 cf tele macro 58mm
tamron 2x teleconvertor
tamron 2.5 90mm
nikon fg
nikon f501
nikon e series 36-72 3.5
nikon 50 1.4
formula 5 52mm skylight1
makinon mag +1+2+4
cherry 58mm delta? pl
" 52
sigmo alpha 3.5-4.5 35-135 phi55
vivitar 2x macro teleconvertor
tamron sp 3.5 70-210 cf tele macro 58mm
tamron 2x teleconvertor
tamron 2.5 90mm
The stuff in bold is something you really could use (depending on the Mounts)
Get yourself a Nikon D80 to autofocus the 50 f1.4, not sure about the Tamron’s.