Help! Need to add length to a finished afghan panel

I’ve been knitting the following striped three panel afghan, and after I cast off the edge of the first panel, I realized that my calculations were off when I lengthened each stripe, and now three inches needs to be added to both the third and last stripe. For the last one, I’m just going to undo the cast off edge, but I have no idea how to add length to the third stripe.

This is the pattern I’ve been using: https://www.michaels.com/on/demandware.static/Sites-Site/Sites-siteCatalog_michaels_US/default/dw373a3ead/project_pdf/72951.pdf

Welcome to KnittingHelp!
Why can’t you undo the third strip cast off and ribbing? Then you can extend the knitting for however many more inches you need.

The third stripe is in the middle of the first panel, so it isn’t along a cast off edge. Somehow, I have to detach it from the other half, lengthen it, and then reattach it.

This afghan looks pretty free-form to me. It seems that it wouldn’t be remarkable to have a shorter block of color in the middle and an extension of the cast off end. You could even add a block of a different color at the end.

However, if this isn’t your preference, you can cut the knitting and extend the block at the center, then graft the ends together. It’s not simple but it will work.
I [I]strongly[/I] recommend trying this out on a swatch first rather than cutting into your long strip and regretting it.