Well, selfishness is a fascinating concept to think about in this play. To what extent are the principal characters selfish in what they do and how they act? Certainly there does seem to be an element of selfishness in Romeo and Juliet's actions as they sacrifice everything else to pursue their love for each other, no matter what the consequences.
Given this, there might be a number of possible contenders for the prize of the most selfish character in this Act. Let us remember it is in this section of the play that Romeo kills Tybalt and is exiled, and Romeo and Juliet have one night together as man and wife before he must flee Verona. We could argue that Romeo is most selfish. He after all kills Tybalt knowing full well the consequences of this action and how it would impact both himself and Juliet.
However, we could also argue that Lord Capulet is the most selfish character when he insists that Juliet will marry Paris:
Thursday is near; lay hand on heart, advise.
And you be mine, I'll give yo to my friend;
And you be not, hang, beg, starve, die in teh streets,
For, by my soul, I'll ne'er acknowledge thee,
Nor what is mine shall never do thee good.
Even though he doesn't know that Juliet is already married, it is incredibly selfish of him to force his daughter to marry so soon after her beloved cousin's death and to issue her with such terrible threats. Therefore, I personally would argue that Lord Capulet is the most selfish character in this Act.