Thing is, an April release for all those devices falls in line with when the Droid Incredible was released, which was around the same time last year. No matter what, that's not gonna play a huge factor in sales for the EVO 3D. If they do manage to steal away some customers that are going to be getting off contract w/ the other providers soon, those people will just continue month to month service until Sprint releases the EVO 3D. Ultimately, the general consumer is going for convenience and savings, while still being able to get the "best phone" for their particular needs. If they believe that Sprint will adhere to that more-so than their current provider, they'll leave. If not, they won't.
I know for sure that I would've left Sprint a while ago, if they weren't the only provider that worked at my house and also the most affordable bang-for-the buck plan pricing. Truth is, they are...for my needs...so I'll continue to be on Sprint until that changes.