P1.distance = (Dot.x - P1.x)^2 + (Dot.z - P1.z)^2
P2.distance = (Dot.x - P2.x)^2 + (Dot.z - P2.z)^2
P3.distance = (Dot.x - P3.x)^2 + (Dot.z - P3.z)^2

TotalDistance = P1.distance + P2.distance + P3.distance

P1.weight = P1.distance / TotalDistance
P2.weight = P2.distance / TotalDistance
P3.weight = P3.distance / TotalDistance

Dot.y = (P1.y * P1.weight) + (P2.y * P2.weight) + (P3.y * P3.weight)