Which pluggin

Do you need a plugin to make this scrolling effect for testimonials? if yes which one do you recommend?

You should be using a block library plugin like Spectra or Stackable, those will give blocks that you could you for something like this.
Either a testimonial block, a slider, or a carousel

Which theme are you planning on using?

OK thanks. using blocksy theme.
I dowloaded the ‘gutslider’ pluggin but I can’t figure out how to get all three testimonials showing at once so maybe I’ll try a block library instead.

Reminder for anyone looking here; we have some recommendations for plugins here: Client site: Tools - Google Sheets