MTS

You say the measurement across the center - tip to tip - is 3" unfinished.

That's all the information you're giving, and it's not enough to answer your question.

Here's a picture of diamond.
Are you trying to make it into a rectangle like this?

What is the height of your diamond?
Where do you want the square to go?
Is it just a plain block next to this one?
Is it a 60 degree diamond?
What did the instructions for the original quilt say about finished block sizes?

And a picture of (or link to) the original or 2nd quilt pattern would be very helpful.