Once you have both the cmake and the cmake3 package installed on your machine, you can use update-alternatives to switch between both packages.
Use the alternatives command to register both installations:
$ sudo alternatives --install /usr/local/bin/cmake cmake /usr/bin/cmake 10 \
--slave /usr/local/bin/ctest ctest /usr/bin/ctest \
--slave /usr/local/bin/cpack cpack /usr/bin/cpack \
--slave /usr/local/bin/ccmake ccmake /usr/bin/ccmake \
--family cmake
$ sudo alternatives --install /usr/local/bin/cmake cmake /usr/bin/cmake3 20 \
--slave /usr/local/bin/ctest ctest /usr/bin/ctest3 \
--slave /usr/local/bin/cpack cpack /usr/bin/cpack3 \
--slave /usr/local/bin/ccmake ccmake /usr/bin/ccmake3 \
--family cmake
After these two commands, cmake3 will be invoked by default, when you enter cmake from a bash prompt or start a bash script. The commands also take care of registering a few secondary commands like ctest which need to be switched along with cmake.
If you need to switch back to cmake 2.8 as the default, run the following command:
$ sudo alternatives --config cmake
There are 2 programs which provide 'cmake'.
Selection Command
-----------------------------------------------
1 cmake (/usr/bin/cmake)
*+ 2 cmake (/usr/bin/cmake3)
Enter to keep the current selection[+], or type selection number: 1