Installing 3 Eve Light Switches - one works, but two others just flash the lights every 6 seconds
You add to the neutral pile instead of splitting it up

From your description, it sounds like you're picking a specific neutral wire out of the neutral bundle to attach the neutral from the smart switch to.  This is wrong, as it disconnects that neutral from the rest of the neutrals, leaving the smart switch possibly unable to power itself properly, as well as some loads unpowered as they have no return path to the utility for electric current to flow.

What you need to do instead when installing a smart switch is keep all the existing neutrals together while adding the smart switch's neutral to that bundle.  That way, the current that powers the smart switch joins with the rest of the current on that circuit on the way back to the utility.