Yes, like 70sSanO explained above.

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It's quick and easy to shift the back 7 cogs. Shifting the front takes a bit more time and effort. So the three sets of gearing overlap to help minimize how often the front needs to be shifted.

I don't know your gear tooth counts. But here's a typical example for 3 front, 7 rear gears.
This example:
Front 24 small, 36 middle, 48 biggest.
Rear: 13, 15, 17, 19, 21, 24, 28.

This chart shows typical pedaling speeds for each front-rear gear combination when cruising along. "RPM" is the same as "cadence", the number of pedal revolutions per minute.
To understand this, see the black chainring bar labeled "15" as an example. That's the middle chainring with second to smallest cog in the back (a 15 tooth cog). At 70 rpm, it's about 13mph, at the faster pedaling 85 rpm, it's about 16 mph.

(some of the charts posted above are a bit more technical -- I like this visual style to make the gearing relationships quite obvious.)


Using the middle ring (in black), it's best at about 7-8 mph to about 15 mph. (It still could be used outside this range, but the rider would spinning really fast at higher mph, or at slow speeds "grinding" with heavy pedal pressures at a slow cadence.)

Just riding along, on flats or with easy grades: The middle chainring is good.
Want to go fast, or there's a slight downhill: The big chainring.
Moderate or steep hills ahead: get in the small chainring. This is great for strong headwinds too!

Look at the 15 rear cog with the middle chainring -- it's near the 15 mph range. Leaving it in the 15 rear and shifting the front to the smallest chainring, that's at a 10 mph range -- it's going to feel "way too easy, can't pedal that fast" if the bike is still going 15 mph. So riders usually shift the front and also shift 2 or 3 gears in the back to kind of match the same road speed. That takes a bit of practice -- so try it at moderate speeds on a flat, quiet road.

With experience, you'll know when you want to shift the front. Shift it before the base of the hill climb, for instance, it's way easier to shift the front when the rider is just easy pedaling. No strong forces on the chain then. And after a few weeks of riding, you'll start to just shift the correct direction, without having to think about it first -- it becomes automatic. "I'll be going slower ahead" -- click click and the correct gearing is ready.