Here’s the whole progression below. Notice there is overlap – you can sometimes get nearly the same effect by changing to a different front chainwheel and different rear cog.
But I can tell you from decades of riding, nobody maniacally shifts around to these combinations in order.
IN GENERAL, people ride long periods shifting only the back.
1)
On flat ground, you probably stay with the middle chainring in front.
2)
Up hills or into a strong headwinds, switch to the small chainring in front
3)
Trying to break speed records or racing, you use the big chainring in front
Front….................Rear….........Rear wheel turns per chainwheel turn
28….................27….................1.04
28….................25….................1.12
28….................23….................1.22
28….................21….................1.33
38….................27….................1.41
38….................25….................1.52
28….................18….................1.56
38….................23….................1.65
28….................16….................1.75
48….................27….................1.78
38….................21….................1.81
48….................25….................1.92
48….................23….................2.09
38….................18….................2.11
28….................13….................2.15
48….................21….................2.29
38….................16….................2.38
48….................18….................2.67
38….................13….................2.92
48….................16….................3.00
48….................13….................3.69