I'm trying to join agents to my 3 node HA cluster by doing:

curl -sfL <https://get.k3s.io/> | K3S_TOKEN=<TOKEN> sh -s - agent \
--docker \
--server <https://a.b.c.d:6443> \
--flannel-iface eth2

However, they end up having the same roles as the control-plane:

control-plane,etcd,master

What am I doing wrong?

how are you deleting? https://docs.k3s.io/installation/uninstall

My first delete I had to use the k3s-uninstall.sh script, which implies to me something happened to treat it like a controlplane host. After I found that wasn't enough, I removed it completely with kube delete node and by doing that I was finally able to rebuild it as an agent.