Can anyone explain why in the solution they are diving by 3? Shouldnt it be weight times duration and then add up each bond’s duration contribution to portfolio? Where is this divide by 3 coming from? I can’t back into their number. Thank you!

I think you are correct and the division by 3 is just wrong. (assuming you are talking about the CFAI AM mock).

the divided by 3 in the formula was ignored anyways