Old plumbing drains right into the ground

It would have to be vented through the roof or with an auto vent. Neither of which would relieve a backup.
I was thinking the wye is below the slab, where it would just overflow into the dry well if there was a blockage.
Guy I knew maybe 25 years ago, lived in the middle of nowhere with his wife on a farm. They were pretty broke.

Their septic tank and field were shot. He hired someone with a trencher to dig a trench from the house to a creek, maybe 250 yards away.
Got the trench dug, paid the man and said "I will handle the rest"

Ran his own pipe, dumping directly into the creek and filled in the dirt. Probably still doing it that way, but I have been there in years. No septic tank or anything. Direct route from house to creek.
If you are saying a double wye where one side would be like a vent to the atmosphere, that would be a good backup solution. That way one end points up and if the pipe couldn’t handle the flow, it would just overflow out the top.

Avacado was talking about adding another drain access. A double wye would work if the other drain pipe was coming from another direction.

You would have some serious problems if the overflow was going out the vent.
When I was a kid, one of my friends lived in a house that had a kitchen sink that spewed water onto the back vegetable garden. In the winter that water created a skating rink that we all enjoyed. It sloped a bit but that didn't seem to bother anyone.

My niece lived in Sydney, Australia for a while. She and her husband owned a house there where grey water was simply discharged into the back yard. They sold that house for over $1,000,000.
A word of warning. If you bring a licensed plumber in, he/she is obligated by law to bring your entire waste plumbing system up to current code. There is no such thing as "grandfathering" when it comes to this sort of thing. This could get VERY expensive! Don't even bring a plumber in to look at it, depending on local ordinances he/she might be obligated to report it to authorities. As a homeowner, you can make "repairs" yourself, even if they are not to code.
The 3" line will handle both of the washing machine's water without a problem. I do not recommend dumping this water into the "hole".
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