What flowers can I plant now (September) in Zone 3?

For starters, you need to add compost to your clay soil, or you will be disappointed at the results of your tree growth and any flowers or bulbs you plant around it. Clay soil won't allow the water to drain away from the plants and will slowly drown the tree and anything else that needs some air to be healthy. Adding compost at the top of the soil will help the roots of flowers or bulbs, but the tree needs to be re-planted after digging compost into the planting hole. It's an expensive lesson in gardening when you buy pricey plants and don't provide the proper environment for them to grow.