To make Quake3 loop...
Make a file in the Quake3 (or whatever folder you installed Q3 into) folder called demoloop.bat
Put the following text in demoloop.bat:
quake3.exe +exec demoloop.cfg
In the baseq3 folder make a file called demoloop.cfg and put the following in it:
timedemo 1
set demoloop "demo demo001; set nextdemo vstr demoloop2"
set demoloop2 "demo demo002; set nextdemo vstr demoloop3"
set demoloop3 "demo quaver; set nextdemo vstr demoloop"
vstr demoloop
You probably don't have the quaver demo and so either download it or remove the 4th line in the demoloop.cfg and edit the 3rd line so that at the end it sayz "vstr demoloop".
To run the loop just execute the demoloop.bat