(For those who don't know, the EVSE that GM delivers with the Bolt and 2nd-generation Volt is able to handle 240v, although it's not advertised as such, and the physical plug is a hard-wired NEMA 5-15, so it won't work at 240v without a hackish custom-built adapter. Some people use such adapters to plug it into a 240v outlet so as to double the charge rate compared to standard NEMA 5-15 charging.)
Most likely you'll need to buy
a $35 adapter to plug into your 240v outlet. It's conceivable that the hackish adapter you've got would work with the Model 3's Mobile Connector and its NEMA 5-15 adapter to charge at 12 amps and 240v, but there's also a chance that it won't work or that you'd fry the Mobile Connector. I don't recall hearing of anybody who's tried it. Personally, I wouldn't risk it -- not to save $35. You're spending tens of thousands of dollars on the car, so don't cheap out on this. You may even be able to add the adapter to the financed price, which would work out to a buck or so a month.
I'm afraid that
@Enginerd is providing outdated information. Although Tesla
used to provide a NEMA 14-50 adapter with the Model 3, that adapter was removed from the standard kit a while ago (about 4 months ago, IIRC). Thus, unless you happen to buy a car that was produced before that date, or somehow otherwise luck out, you'll need to buy an adapter. It's unclear from your description,
@Turlejay, what type of outlet you have. It might be a NEMA 14-50 or something else. Chances are it's one that Tesla supports, but if not, you'll either need an adapter in addition to the Tesla adapter or you'll need a third-party product. Post a photo of your outlet if you need help identifying it.
Note that the adapters for the Gen2 Mobile Connector that comes with the Model 3 are "smart." Each one is encoded with the amperage it supports, so you can only draw 12 amps through the NEMA 5-15 adapter, 24 amps through a NEMA 14-30 adapter, etc. I don't know offhand if the voltage is also encoded and limited.
Also, I'm sure somebody else will want to note, so I'll save them the hassle: 220v is not a standard voltage in the United States. Most homes have 120v and 240v, each with an acceptable variance, so you might actually measure 220v in a few cases, but it's nominally 240v. Some multi-family and commercial sites use 208v rather than 240v.