Let's see
is definitely powerful, her lore describing her as constantly growing in power, most likely by now she is on the same magical power level as
, also her lore hints at her sapping the magic out of the Ghost Willow, which is one of the ancient trees of Ionia with immense magical power in them, sounds a lot like the God Willow right? So at the very absolute least and to some extent Syndra's power is at a similar or higher level than 
on the other hand is ancient and powerful in her own way, as she has Dark ice wich is basically the edgy version of True Ice, and her powers where gifted to her by The Watchers, Godly beings that are worshiped by her and The Void born (makes you wonder if that means
and
would become allies) so technically she is like an aspect but for the Watchers instead of the Targonians, this fact alone puts her magic powers up there as well
is the oldest or second oldest contestant here, depending on if the Frozen Watchers' arrival to Freljord happened before
's rise or After his Fall, but she definitely has enough knowledge and experience to pair up with her guile and powers to keep up with the other two, and the Immortal Bastion definitely had enough dark magic/relics related to Necromancy and other forbidden techniques for her to take advantage of
If it was a straight on battle at a moments notice
vaporizes
but fails to break through
's defenses and probably gets frozen
If we are going for a long term war then
still loses, but with time
could possibly grow strong enough to break through Dark Ice and vaporize 
If it's any sort of battle, whether single round or full out war, if they get time and knowledge of each other to prepare with then
dominates both, especially since her magic is more flexible than
's and
was bound before for many years