The easiest way is to employ usual Spring Security authentication (e.g. form-based with username/password) and enable the remember-me feature.
As long as all three applications are deployed on the same domain (and can therefore share their cookies), the remember-me feature will work seamlessly for all of them, i.e. once user authenticates at any of the applications she will be able to access the others without need for re-authentication.
This of course doesn't address issues like single logout, but that doesn't seem to be your aim anyway.
You can set this up in a short time and don't need and third party SSO components. Although relaying on standard SSO protocols and dedicated technologies tends to be a more secure option.