my dd has started and had her first lesson just before turning 4.I think they like them to be four at the stables,but I would think a sensible three year old would be fine.I started Ballet classes at 3 for instance.
Hi, at our local riding school they will do 1/2 hour lessons for children aged 3 and up, but there is another one a bit further away that have taken my dd since she was 2 for a 10/15 minute ride up a path and back (she loves it!). At 5 they start Saturday clubs where you can leave the kids for half a day if you want to. I've found places on holiday who are happy to take children really young so long as you sign a disclaimer. Have a look on line at your local riding schools and they will probably list what they do but I think 3 1/2 should be fine if our local ones are anything to go by.