Can someone let me know what is the difference between Bootstrap, Twitter Bootstrap and Bootstrap 3?
In the context of CSS frameworks, Bootstrap and Twitter Bootstrap are the same thing.
Bootstrap 3 is the third major version of Twitter Bootstrap. (When this question was asked, it had not been released yet.)
(There's another unrelated definition: to bootstrap a computer program or system can refer to running certain phases of early startup. It's often abbreviated as "booting", as in "booting up a computer". Wikipedia has an extensive definition and etymology which I won't reproduce here.)