I use Wolverine Brass tank bolts. They have a nylon reinforced rubber washers and two sets of nuts and heavy washers for each bolt. All heavy brass. They have large bolt heads that almost cover the entire washer....not allowing the bolt head to cut into the washer. I use one rubber washer under the bolt head,slip the bolt through the tank and secure with one brass washer and one nut. Repeat for the second bolt. The install the tank on the bowl. Now one rubber washer under the bowl,one brass washer and then the nut.
Mansfield still using three bolts?