7,5,3,6,9
what three numbers add up to 12

Explanation:

3+3+6=12 is the three numbers adding up to 12

a:

by the combination of an unlimited number of elements

each chemical family is represented as a group or column on the periodic table. based on chemical properties, there are 6 types of chemical families. group 1 represents alkali metals, group 2 represents alkaline earth metals, groups 3 to 12 represent transition metals, groups 13 to 16 represent non-metals and other metals, group 17 represents halogens and group 18 represents noble gases.