Fourth, twenty-fourth and twenty-fifth letters, all going to the third, reportedly? (6)

I haven't cracked this one yet — but the brilliant team at Fifteensquared have:

Rivers Forth, Exe and Wye (reportedly fourth, X (twenty fourth letter) and Y (twenty fifth letter) all go to the sea, reportedly C (third letter) Edit: thanks to Paula @5 – it’s, of course, DEE (fourth letter)