Finding which gear on my bike is closest to a single-gear bike
We can't tell without more information about your gears.

However, you can figure it out for yourself quite easily, by looking at the ratio of chainring teeth to rear sprocket teeth on the single speed bike (the most common I've seen is 44/18 = 2.44).

Then you just need to figure out the size of your current bikes chainrings, the size of the sprockets on your current bike, and calculate all 21 gear ratios, and find which is closest to the single speed ratio.

If I had to guess, I would say it would most likely be equivalent to your middle chainring (39 teeth?) and somewhere near the third smallest rear sprocket (16 teeth?) because 39/16 = 44/18.

Of course, you can almost always pick and choose the gearing on a single speed bike, so realistically you yourself get to pick "what gear on the 21 would be for a 1".