Deleting Specific Values From Textfield...

I have a form that has several disabled text fields. In these text
fields, I have initial values. Next to each text field lies a "copy"
button. When clicked, the content of the disabled text field is
appended to the content of another textfield, the only one that isn't
disabled (therefore the user can copy content from the other text
fields, and also add what they want). I also have a remove button,
which clears the content of that one non-disabled text field.

Pretty easy so far.

Here's the problem. Imagine I copy the content from textfield 1 into
the blank text field, then do the same for textfield 2 and 3.

Between each "insert", I've added a comma. So the non-disabled
(active) text field would have the following content:
content1, content2, content3,

Now say that the user wants to remove content2, without removing the
rest, how could I go about doing this?