Q1- For Abraham, his justification was when God accepted him as righteous which came as a result of his faith and believing in God.
Q2 - For us, our justification or being put right with God comes as a result of our faith in Jesus Christ. This means that we gain our righteousness through Jesus Christ.
Q3 - For me, justified means to be vindicated in your beliefs or actions. In this context, I think it means too be put right with God or to be considered in the righteous by God.