Unlike GenRex, I honestly found the first puzzle quite easy! It wasn't kidding when it said to ignore information that wasn't relevant because it would try to confuse you. The second one wasn't bad either.
The third one I couldn't do, but it's not because of anything I did wrong: first of all, I think it's supposed to say the left side weighs 210 lbs, not 120. 120 would mean each silver weighs 40 lbs, but they actually weigh 70. I also think you have an error in your initial value for the left side; the change variables command in the event sets it to 21, but I think it was supposed to be 210. I'm not actually sure it's possible to equalise the scales with things the way they are.
It looks like you may have changed the value of a silver partway through, as having a value of 7 *does* line up with the clue for a harp (1 silver + 3 books). But in that case a book becomes 1.7 lbs rather than 8.
Not a bad little set of puzzles, though! Definitely got me thinking.