I use a single book ring through holes in the top left of my cards. My ~40 card stack is for everything -except- my calendar. I use a commercial month and week view pocketable size for that. I have a thin black plastic cover 'card' front and back cut from a larger report cover. I put a hair band around them to keep the stack stable. Pocketable in pants front, back or suit coat.

You can bind your 3 x 5 cards together in many different ways.

The book ring and band method is simple and inexpensive. And the cards can be rearranged easily on the ring and the whole "mechanism" costs next-to-nothing.

Binding ties also work, there's an example somewhere here on the site. These are especially good if you want to have 2 or more holes in the top of the cards, and don't want to bother with the Circa system. (Which is a very expensive set up if you're only using it for 3 x 5 cards).

If you want no holes in your cards, then a elastic band or hairband works (with a small paper folder fif you want to protect the cards).
Or an envelope.
Or push on binder clips for a more solid binding.

If you like flicking though your cards for reference, then a cheap photo-album is a good alternative. (One of those 3x5 plastic sleeve albums sold at discount shops works well).

My personal favourite is a small binding ring.
It allows easy 'flipping' through the stack of cards but it hasn't got the rigidity of a spiral bound notepad (one of the problems with the Circa approach for me, if I wanted a top or side bound notepad I'd buy one).
The cards can be spread out like a fan if needed, flipped like a shorthand notebook or (if you have a lot of cards) stored on multiple small binder rings by topic.
And, the cards have only one small hole in a corner.