Answer: It's not too late as long as the tubers are still firm and plump. (If they are moldy or rotten or totally dried up and shriveled they will not grow.) They will probably take a few months to start blooming, though. Use a good quality soilless potting mix formulated for container plants, set the tuber about a half inch deep with the dent on top. I would put one per six inch pot. Water to settle the soil and keep it slightly damp (not wet) until it sprouts -- it may take two weeks or more to see growth. Keep the pot in a warm bright location but out of direct sun until it sprouts, as soon as it sprouts you can set it outside. As the summer gets warmer you may want to shade it starting about noontime or so. Here are more detailed instructions you may find helpful as well. You may need to cut and paste the complete url into your browser to make it work correctly.
Good luck with your begonias!
Q&A Library Searching Tips