How to modify a surface skimmer to 3” to use gravity to your filter 

nice video keep them coming

Did a similar mod to mine last year, I used 110mm hole saw which removes both factory holes completely but still a good enough lip for structual strength, I fitted a reducer boot that fits perfectly around the bottom of the skimmer then reducing down to take a 110mm slide valve as I converted mine to gravity feed an ultrasieve. The only issue with making this a gravity feed skimmer is the floating weir with basket will no longer work as the pumps suction is needed to draw the floating weir down to allow water to flow, plus never really like the floating weir as it always had an issue with drawing in air and burping all the time, I just bought the traditional weir flap for it and that works way better IMO plus no real limit in regards to flow rate where pump fed I believe is around 8000LPH, good skimmers though as mine was fitted in 2005 and still going strong

Why not just use a 4" bulkhead?