George,
The numbering is generic to any key. #1 would be the chord for the key you are playing. The othger #'s are the chord progression and they would count the same as notes on an 8 note scale.
If you are playing in "C" your scale would be CDEFGABC .
For a 3 chord progression you would play C,F,&G aka 1,4&5.
Using these #'s you can choose any key and just play the 1st, 4th,& 5th chords .
Slainte ,
Happy New Year!
       Sandy