Why is unequal access to basic service amount to social injustice? at least three points may be used for this discussion

Okay, so - 

This is a discussion you must do on your own. We can't do this for you, but we can set you on the right track.

Equality is very important to maintaining social justice. 

If things were to be completely unequal, that would be disvaluing a member or group of society as 'worse' than someone else. This is not justice, this is discrimination.

Another way it is injustice is denying some service simply because someone possesses a property, which most often is irrelevant to deciding who they are as a person.

Hopefully, this helps! =)