Multiple Ordeals of Nylea on one creature
The basic answer is that the creature gets 3 more counters (for a total of 5), you sacrifice all 3 Ordeals, and you search your library for 6 basic lands and put them onto the battlefield tapped.

Step by step, here's what happens:


You attack with the creature.
Abilities that trigger when you attack are put on the stack. In this case, that is the first ability of each of the 3 Ordeals attached to the creature.
The first ability of the first Ordeal resolves. You put a counter on the creature. It has 3 or more counters, so you sacrifice the Ordeal. The Ordeal's second ability triggers, and is put on the stack.
Each player gains priority, and has an opportunity to respond.
The second ability of the first Ordeal resolves. You search your library for 2 basic lands, and put them on the battlefield tapped.
Each player gains priority, and has an opportunity to respond.
Steps 3-6 repeat for each of the second and the third Ordeal. In each of those iterations, you put another counter on the creature, sacrifice that Ordeal, and search for 2 more lands.