How I think a third trinket slot could work

First it would need to be something you unlock since a third slot would be very op which means we need some sort of barrier. The best way I can think of to do this is to make it so the option to buy it with Ichor appears in Dandy's shop after getting 100 percent research for both lethals (Dandy and Dyle)

Now we come to the second problem, loadouts. If it works the same as a normal trinket slot it can be easily abused for combos like veemote/participation/mag or dog plush/speedy shoes/ and either ribbon spool or clown horn. Solution, you can only have two items of a certain kind equipped. For example if you have mag/participation equipped you cannot equip veemote, or if you have dog plush/speedy shoes you cant equip a third trinket that affects speed. Would there still be overpowered combos? Yes, but this would help nerf that issue while still allowing awsome loadouts.

Feel free to suggest more ways this sort of thing might work.