On the table there are $20$ coins that carry the letters $A,B,C,\ldots,T$. The values of these coins are $1,2,3,\ldots,20$ Euro (but the assignment of values to letters is secret). Fredo knows the value of every single coin, while Cosmo does not know.
In every round of the game, Cosmo must point at three different coins on the table (which we call the first, second, third coin). Fredo pockets the first coin, and then tells Cosmo truthfully which of the other two coins has higher value. The game then goes on with the remaining coins.
Cosmo may end the game after any round, by pointing at two coins still on the table and announcing that their total value is at least $28$ Euro. Cosmo wins the game if and only if his announcement is correct.
Question: How can Cosmo enforce a win?