I would not start with all three at once. Varnish is usually for full-page caching, so it helps when your public pages get a lot of repeat traffic. Redis and Memcached are more about object caching, which helps reduce repeated database work.

For a normal WordPress site, I would start with good page caching and a CDN first. If the admin area, WooCommerce, search, or logged-in pages still feel slow, then Redis object caching may be worth adding.

The easy way to think about it is this: Varnish helps serve pages before WordPress does much work. Redis or Memcached help WordPress remember database results. So if your public blog posts are slow, page caching or Varnish may help. If your site is database-heavy, Redis may help more.

I would check server response time, database queries, and whether the site is mostly static or dynamic before adding more layers.

For my WordPress site, Redis made more difference than I expected because the site had many plugins and dynamic elements. But for a simple blog, regular page caching was enough.

I think the sign you need Redis or Memcached is when pages still feel slow even after image optimization, CDN, and page cache are already working.