My wife’s hybrid plug-in gets 85 miles per gallon. Every night, we plug the car in to recharge the battery and get 27 miles off a full charge. Then the gas engine kicks in, or whenever needed. Sometimes we can go 800 miles before needing another tank of gas (10 gallons). My question is one of oil change frequency. For my Cadillac, I change the oil every 5K miles due to short trips I put on the car. My wife’s dealer wants to see her bring in the car every 3K miles for an oil change. I’m thinking it’s more like every 7.5K miles. What formula would you use to figure how many total miles are actually put on the engine in order to know when to have an oil change?
Go by the schedule in your owners manual. Chances are oil change will be noted in time or mileage WHICH EVER COMES FIRST. Or your car may have an oil life monitor which will tell you when to chain oil.
Your dealers’ recommendation sounds bogus.
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Your dealers recommendation sounds like he has a boat payment coming up! 
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There’s more to an oil change regimen than miles. There’s time, environmental conditions, driving habits, and so on. I would never go over 5k miles and in some cases 3k is justified.
OIl changes are not a money making proposition for any shop. They’re a nuisance IMO. If you’re standing around with nothing to do then I guess an oil change is better than nothing. Speaking from an earning money by the mechanic standpoint.
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Given the driving habits with so many of the miles being under electric power, it sure does sound even more bogus than suggesting that frequency for a gas-only vehicle. But the OP hasn’t told us how many miles this vehicle is driven per year.
The factory oil change interval for you is 7,500 for ‘normal’ driving conditions, 3,750 for ‘severe’. Which are you?
Your dealer, as others said, is either honestly mistaken, or just trying to generate more business by selling you unneeded service. Generally I agree simply going by the owners’ manual … which I’m sure doesn’t say you need to change oil at 3000 miles… is fine. There are some exceptions: For example, the manual will typically say something like "every 6000 miles or every six months whichever comes first. Now that “or every six months” idea is based on their assumption what if you put on less than 6000 miles in half a year you’ve been driving short trips. And short trips are harder on the oil because it doesn’t get hot enough to drive out the moisture, etc, etc. But if you have an unusual pattern of driving, like you drove only 3000 miles in that 6 months but it was mostly all on relatively long trips of, say over 30 minutes or 60 miles, where the engine had plenty of time to warm up properly than there’s no reason to change the oil that soon. What about if the car just sat in the garage unused for 6 months since the last oil change, and the oil is basically completely new? It would make no sense to drain it out just because the calendar crossed the six months mark. I once asked a service manager who was pushing the “change oil every six months” idea this: “So if your dealership had a car sitting unsold in the sales lot for six months undriven, would you lean on them to change the oil?”