Due to voter concentrations, the CPC will outperform the LPC and probably even NDP in seat count at the same federal vote %. So in order for the CPC to fall to 3rd you'd need to see them at below 25% national support and the LPC at 30% or above, I'd think, which is not very likely considering the very strong CPC base. The CPC is very concentrated in western seats and suburban Ontario seats, much in the same way the NDP is concentrated in Quebec, BC, and urban seats. The Liberals are less concentrated except in the Maritimes.
As for what is best, I'd honestly this is my dream:
- CPC wins election with minority, but with less votes than NDP.
- NDP is second, and LPC third, but with a total seat count exceeding the CPC.
The reason I want that exact scenario is this: The NDP and LPC would likely form a coalition government, or at least an LPC-backed NDP minority, and that government would likely push for democratic reform. It would also normalize the idea of a coalition governing the country, which would be needed in a PR or MMR system.