Something like the following:
conda create -n jupyterlab -c conda-forge jupyterlab=3 -y
conda activate jupyterlab
# open JupyterLab
jupyter lab
You might also want to check the order of the configuration folders returned by the jupyter --paths command. There is more info about this in: Jupyter Paths priority order
Just to check whether if there is a page_config.json file in PREFIX/etc/jupyter/labconfig/ (or in ~/.jupyter/labconfig) that might have the terminalsAvailable setting disabled.