Should I main Irelia riven or Fiona for top lane secondary role?

Depends what you want to do.

Of the three, Irelia is usually the one that I'd most like to see on my team, because she's a solid and reliable pick, decently tanky so she fits most team compositions, and can do quite well even if behind.

The others are much more hit-and-miss. They can carry hard if they do well, but can also become almost useless if they fall behind. They are also squishy, so they don't fit into every team (e.g. you probably shouldn't play them if your jungler is a squishy.)

I don't play top-lane much myself, so I won't give you advice on the actual champions themselves - I'm just saying what it feel like to have them on my team when I'm playing another role.

Irelia is your best choice tbh.

Riven is a 'medium' difficulty champion, however, her skill ceiling is so high that anyone who masters her is in challenger (lol). She has low CD's, AOE abilities, 4 dashes, a shield, a knockup and a stun (can jump over walls as well). Her kit is over loaded in other words. Just like lee sin though, to utilize her kit at the highest level requires the person to know alot about the game and the champion. She requires alot of practice to obtain muscle memory when playing her.

I do not know much about fiora because I have not played her much, but the times I have played her makes her seem relatively simple and straightforward to play.

Irelia is somewhere inbetween these 2. It is harder to utilize her kit to the highest level; more so than fiora, however, she still has a lower skill cap than riven. This is why I suggested irelia, much less of a hastle, but when mastered she is still a very terrifying champion to play against

Probably Irelia is the best choice.