I can also nitpick that the parentheses are in the wrong place. It's not that the conjunction of A, B, and C must be reliable, but that each of them individually must be reliable (as is indicated in English by the use of ‘are’ instead of ‘is’). So first you should make it all symbols by introducing modal operators for the usage of a fact and for the reliability of a fact, then group properly to get U[A] ↔ (R[A] ∧ R[B] ∧ R[C]), not U[A] ↔ R[A ∧ B ∧ C].