Can anyone please tell me what year this Miyata 3 speed is? I have searched the internet with no success. It has a Shimano 3S rear hub.

The serial number "I" prefix would indicate 1980 frame age.The lower case "t" in the 'Miyata' logo would indicate pre-1981. The USA-market equivalent to this was called the "Commuter", beginning here around 1982.

It matches up pretty closely to the 3-speeds in the 1981 & 1982 catalogs, except for the chain guard. The '81 has brown grips and seat. I don't see the three speeds mentioned in other years. If the first character of the serial number is the letter i, that might indicate 1979 (correction - 1980), but I didn't see any three speeds mentioned except for '81 and '82. It could be the frame was built in late 1980 for the '81 model year.
It looks like a vary nice three speed at that, with alloy rims (27"), alloy bars, stem, crank, chrome moly frame. Way above your average three speed.

Link to Miyata catalogs: https://www.ragandbone.ca/Miyata/miyata_selector.html

I agree, it looks very close to the 1981-1982 Commuter bikes. The differences are, the chain guard, mine does not have a front carrier, my generator set has the light on the generator not the fender, and lastly, mine is a 10-24 steel frame. I have only heard of the Shimano 333 hubs. Does the Shimano 3S get the same bad rap as the the 333?

So, I guess, it is a 1980? Per the "I" in the serial number, the lower case "t" in Miyata, and the fact that it does not exactly match the 1981-1982 brochures.

The 333 has a bad rap, but other Shimano hubs are quite good.

I usually ride Sturmey Archer AW 3 speeds. I did notice with the Shimano 3S, that the gearing ratio must be different than the Sturmey Archer AW, as I normally am never in 3rd gear with the Sturmey Archer , but I have been using 3rd with the Shimano 3S.

Specs on real-world bikes often mismatch with the catalogues. Nice bike,

Not sure about the 3S, but older Shimano 3 speeds (3SC, 3CC, 333, F, FA) are all .75, 1, 1.33, just like an AW. Nexus hubs are a little different at .73, 1, 1.36, which is pretty close.

If you have a smaller chainring in front or a larger cog in back, it will lower the gearing, so you may well be in 3rd when cruising.