the urchin will be best in the largest tank provided the trigger and puffer don't go after it (which i don't think they will); they need a lot of room to scavenge for food so you may have to supplement it with some seaweed or something.
im not trying to attack you, just asking a question, but do you research anything before you buy it? it seems like a lot of times you just come home with stuff which is not suitable for your tank so if the LFS is suggesting these things perhaps you need to verify them here or in a book before you buy