zone3 if a perennial is planted in a container, when or can it be planted in the main garden. is zone3 too cold for a fall re-plant.
A perennial can be moved from a container to the garden any time during the fall, with enough time before the ground freezes to give the plant’s roots a chance to establish themselves in the garden soil.
In your Zone 3 climate, the bigger question is whether the perennial you are planning to move is hardy enough to withstand your winter temperatures in the ground. If it is hardy to Zone 3 or below, your plant has a good chance of overwintering successfully in the garden.
You don’t indicate where you are located, but if you are in Ontario, you may find it interesting to see if there is a Master Gardener group in your area, since advice from local gardening experts is always so practical and useful. Here is a link to the Master Gardeners of Ontario web page showing where groups are located: https://www.mgoi.ca/groups/groups.html All these groups are happy to answer specific gardening questions. If you are outside Ontario, you can google Master Gardeners in your province to find a resource close to you.