Since they are not yet established, I would mulch them. If it is dry in the winter, give them a little water although they will be leafless. They should leaf out late so do not panic if everything else has leafed out except the CMs.
Once established, I do nothing to special to prepare for winter. I have skipped ferts some years too.
Crape myrtles bloom on their new growth, so you can apply a high-nitrogen, lawn-type fertilizer (no weed killers included). The first application should be made in mid-April as the plants are starting to grow most actively. Repeat the application in mid-June for sustained feeding all summer. Apply one pound of fertilizer per cumulative inch of trunk diameter. Add the several trunks together to determine the total needed. Your average date of first frost is around the 2nd week in November so fertilizers, including weak ones like coffee grounds, should be administered 3 months before or more (so June-ish). Your new plants probably came with those round fertilizer pellets so I would not apply any more fertilizers until Spring 2021.
