Imbalanced dataset with 3 classes xgboost scale_pos_weight parameter
For my multiclass classification problem with similar unbalanced data I used the output from sklearn compute_class_weight function:

https://scikit-learn.org/stable/modules/generated/sklearn.utils.class_weight.compute_class_weight.html

sklearn.utils.class_weight.compute_class_weight(class_weight, classes, y)