I've made little activity bags with letters (a tracing page for the older kids names and letter tracing flash cards that use dry erase markers, clothes pin matching for simple words and a couple of alphabet books) and numbers (clothespin matching game for numbers 1-20, some fun numbers books), also some random color matching file folder crafts and clothespin matching that don't really fit in a bag.
I don't do this as a structured activity though, my son loves it but the other little guy really struggles with fine motor stuff so I'm trying to take it slow so he doesn't feel adversive to it at first exposure.

I also don't really do circle time (though I love the idea of the songs mentioned by daycarelady above!), just a story or song or both, and hopefully a craft that is related.

I think the social aspects are more important than the 'academic' side for 3 year olds, like following directions, asking questions, solving problems, and conversing with their peers. I like to develop fine motor skills using games and teaching simple science, math and letters with books and songs.