If it were me and I was out of options, I'd just make one. Buy a PVC adapter for the pipe your connecting to, the one that takes the water away from the house. Then buy an adapter for that which bumps it up to a larger size you need to connect to the metal 3" spout. You're going to form the round end into a square shape for the 3" pipe so you can determine what size round will be enough to make that square using a cloth measuring tape for sewing, was the easiest for me. If you have BBQ grill, fire it up and have yourself a PVC marshmallow roast on a stick and keep rotating. Elongated needle nose pliers or vise grips will help keep control over the piece. Only until it gets soft though, not scorched. It's better to heat it slow than quick, be patient. Ahead of time, I'd be ready with a vice and two blocks of wood to form it quickly before it cools. Squeeze it with the vice and use the blocks of wood to flatten the sides and wait a minute for it to cool or use an air compressor hose for super fast cooling - might be best with two people. If you don't have a vice that big them two boards squeezing with a loop of rope or a nailed hinge block at one end will work too. Am I making sense? Probably not, but hopefully you can pick through that and find something useful. LOL!
I've done this before yrs back, it works. Get an extra or two to experiment and get the feel first. You can always return them. Or experiment with some scrap pipe if you have to get the feel of how it reacts first. You may have to stack two to get the opening you need. It's been a while but I remember it came out good. I bend PVC and Plastic conduit that way all the time for projects where I need a gentle bend or to follow a contour, I use a big plumbing torch usually. But for a big or thick piece, a BBQ will heat it better and all at once more evenly.
Like I said, do it gradually, be patient. When it's ready it'll happen quickly so be alert. You don't have to get it molten or melt it, just softer. It will be hot, wear gloves. Oh and I remember putting a piece of cut off pipe in the other round end to keep it true so it wasn't distorted by my tinkering. Might wanna do something along that lines too. If nothing is available perhaps you could jig saw out a piece of plywood or something for a plug? Just giving you ideas here. Someone no doubt has a better option like a rubber boot joiner or something but if all else fails,.. this is my 2 pennies in a pinch.
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