Julles,
The force of the water is very high. The 3 feet will likely be ok unless you have a rain, which would weaken the wall's capability to hold back the water. Best bet: don't gamble. Make arrangements for your fish to survive in temporary quarters for a week or more and go ahead and dig out the whole pond. Always use existing pond water for temporary fish quarters to minimize ph shock and once the new pond is filled, be sure the new water is ph compatible (and chlorine absent) with the old pond water before reintroducing your fish.
Question: Once the new area is dug out, are you going to install a whole new liner or are you planning on extending your old liner into a new one? While it is possible to seam two liners together, I recommend against it, since if any roots can reach the seam, they WILL breach it (Been there, Done that, Do have the T-Shirt {Complements of Kayak Kid} personal experience over the last 36 years). IMHO, you will be much happier in the long run if you replace the entire liner and save your old liner for another smaller pond in another location.
Tip: have your new liner on hand before beginning the dig. You never know if there could be a shipping delay.
Regarding temporary abodes for your fish: How big and how many? Consider getting some 100 gallon wading pools at Walmart and filling them with water from your pond and putting your fish in them temporarily. You will need to cover them with some sort of net (Bird netting or chicken wire) to keep them from jumping out (They will) and to protect them from predators. Additionally, your fish will require aeration in the pools with an airstone. If kept in the temporary pools longer than 24 hours, some sort of biofiltration (using some media from your regular biofilter) may be necessary. You could accomplish this with 6 gallon buckets and a submersible pump, but be careful about plumbing leaks, etc. ( A rule of thumb: One 6 bucket filter container with media for each 100 gallon container.).
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