Mike Sherman's gear calculator is helpful.

Here's a typical 12-27 cog, 30-39-52 triple road bike gear table.
It shows the kph at 85 rpm cadence for each gear combination. On the web site, you can change the gear sizes and try different cadences, too. Or change to miles per hour.

85 to 90 rpm is a good cadence for many riders. You'll probably have a slower cadence on steep hills, and a faster cadence if you are accelerating.

So the smallest chainring, 30 teeth, is good for speeds of 23 kph down to 12 kph at this cadence (I skip the 12 and 13, since these are to be avoided)
The middle is good for 35 kph down to 15 kph. You can see the three sets have an overlap in their speed ranges.

Notice that the 52-21 is similar to the 39-17 and the 30-13, for example.

------12---13---14---15---17---19---21---24---27
----+------------------------------------------------------
52 | 46.7 43.1 39.9 37.3 33.0 29.5 26.7 23.3 20.8
39 | 34.9 32.3 29.9 28.0 24.6 22.0 20.0 17.5 15.6
30 | 26.9 24.8 23.0 21.6 19.0 17.1 15.4 13.5 11.9