What thresholds determine when a 3-piece lefort is indicated to fix a skeletal open bite, in terms of curve of spee and transverse discrepancy?

Talk: Talk to Orthodontist. He/she will determine extent of your malocclusion and best path of treatment based on more criteria than there is room to discussed here. The Orthodontist will then prepare your occlusion so that skeletal discrepancies can be normalized by you Oral Surgeon. Malocclusions are like snowflakes, no two are identical. There are guidelines, not rules, that determine treatment.