Thanks for the tips, jfollas. So maybe the "3" is a hint that the runes stand for numbers? On this rune website, the rune symbols are paired with numbers. ᛓ = 1,ᚨ=4, ᚹ=3, ᚱ=5. If we look at the placement of the symbols, ᛓ+ᚹ=ᚨ (1+3=4), and if symmetry holds, then "?"=ᚱ=5?