The way I usually do it is that first I sum all the numbers:
1+2+3=6
then I will say that my final ratio has 6 "units" and each unit will be a fraction of the original number:
42/6=7
so each "unit" will have the value of 7
and then I multiply all the numbers by this unit:
1*7=7
2*7=14
3*7=21
so the result will be 7:14:21
