What is the rarity of unique weapons in shadow fight 3?

Of course legendary items has the highest rarity and I have verified it myself. Some of the reasons are:

1. During events the legendary weapons have more rarity.
2. Legendary weapons have better set bonuses.
3. Legendary take more shadow energy to upgrade.

So Legendary has the highest rarity in terms of power in game but you would only get unique items once.

Conclusion: Legendary items has more rarity than unique items.

I'm pretty sure they are equal since both of them have the same amount of slots for abilities.

No , unique rarity equipments are much stronger than the equipments of legendary rarity. Epic rarity equipments also have three slots. Besides , there are few equipments which has two slots instead of three. (For example , helmet of executioner set)

From my experience of gameplay: legendary > unique. You see in special events they tell you to equip weapons of higher rarity as they are more powerful. I remember I had equipped a unique item and the game suggested equipping an epic or legendary item. So there it is.