You have to build it from source. There are no installers for it because it changes daily.
See
http://www.freepascal.org/develop.var and the wiki on how to do that.
You may want to use fpcup/fpclazup from
https://github.com/LongDirtyAnimAlf/Reiniero-fpcup for that.
Or one of the scripts on getlazarus.org.
Note both options are THIRD PARTY and success may vary.
Imho it is easier to learn to build trunk from source once and forget about it. But they can be life savers if there is no suitable starting compiler.
Another option for fpc:
- Download the daily snapshot
- Unpack in a new directory, say name it "fpctrunk"
- goto "fpctrunk"
- run: make clean all install
This only works if you already have fpc 3.0.o installed.
Lazarus is similar but I don't know the exact steps apart from the last (I always checkout the sources with svn)
- create a new directory, say, "laztrunk"
- goto "laztrunk"
- run: make bigide