Idk...seems like it would be a lot of trouble for a mere 3 feet.
I guess it depends on exactly what you mean. Do you mean 3 feet of liner or making the pond 3 feet wider (or longer)?
If you only need 3 feet of liner in order to add a waterfall for instance, you don't need to seam it. You can overlap it without any concerns as long as you have a good overlap. This is how I added my bog to my existing pond. I had a couple of feet of the existing liner that previously lined a waterfall. I placed that existing overlap over the bog/pond wall. Then overlapped the new bog liner on that and extended that all the way down into the pond water. I didn't use any sealant. No leaks at all.
Some people run a bead of PL-S300 caulking between the liners. It comes in a tube you use in a caulking gun.
Adding to a liner certainly can be done, but you have to have the right materials to match your type of liner and the seams need to be totally clean, dry and no wrinkles.
What material is your existing liner made of? EPDM?
You have to figure out if it's worth it, considering the chance of failure. It's no fun tracking down a leak, especially if it's in the dead of winter. Ask me how I know....
Personally, since you're into the existing pond for 12 years, I would wait until that liner needs replacement then go bigger, if that is your intention.
