Yep. Rona actually has more potential in 5v5, since her defense scales with the number of enemies (particularly if she builds SM) and her builds often include BP, which would also scale her damage to the number of enemies.
3v3 is rough for her because 1v1 fights are so much more important in 3v3. She needs to be able to deal with the enemy jungler 1v1, and she needs to be able to ambush the enemy laner and kill them 1v1 — every jungler needs to be able to do those things to work in 3v3. (Note that she doesn’t need to win against the enemy jungler 1v1, just have some way of not losing and to continue farming). Basically, a 3v3 jungler needs to be able to either beat down or escape from Krul, and needs to be able to run down a kiting Ringo. Rona can’t 1v1 and generally can’t escape from other junglers, and she can’t kill carries as well as other junglers. I love the character, but SEMC is too afraid of making her powerful, so she’s weak instead.