Displaying content type as a field in a page
Create a custom block type "Testimonial slideshow" or so (Accessible through local tasks on the block overview page), add a reference field and allow nodes of your given type. Set the cardinality to exactly 3.

Then you can create a block of that type and select 3 testimonials (with select or autocomplete, depending on what you configure on manage display).

That's it, now you just have to implement the necessary templates to show it as a slideshow.