I can’t be arsed to copy and paste it off http://professorlaytonwalkthrough.blogspot.com/2008/02/puzzle131.html

for all three weighings record which side is heavier

l for left

r for right

b if they are balanced

weighing 1

place weights 1 2 3 and 10 on the left

place weights 4 5 6 and 11 on the right

weighing 2

place weights 1 2 3 and 11 on the left

place weights 4 5 6 and 12 on the right

weighing 3

place weights 1 4 7 and 12 on the left

place weights 2 5 8 and 10 on the right

results

lll means 1 is heavy

llr means 2 is heavy

llb means 3 is heavy

rrl means 4 is heavy

rrr means 5 is heavy

rrb means 6 is heavy

bbl means 7 is heavy

bbr means 8 is heavy

bbb means 9 is heavy

lbr means 10 is heavy

rlb means 11 is heavy

brl means 12 is heavy

if none of these is the case then reverse the l and r results from the weighings

the odd weight will match the reversed measurements but will be light rather than heavy