You can use this to figure out the GPU id with the most free memory:
nvidia-smi --query-gpu=memory.free --format=csv,nounits,noheader | nl -v 0 | sort -nrk 2 | cut -f 1 | head -n 1 | xargs

So instead of:
python3 train.py

You can use:
CUDA_VISIBLE_DEVICES=$(nvidia-smi --query-gpu=memory.free --format=csv,nounits,noheader | nl -v 0 | sort -nrk 2 | cut -f 1 | head -n 1 | xargs) python3 train.py

You can also add it into your .bashrc then you can directly run python3 train.py:
alias python3='CUDA_VISIBLE_DEVICES=$(nvidia-smi --query-gpu=memory.free --format=csv,nounits,noheader | nl -v 0 | sort -nrk 2 | cut -f 1 | head -n 1 | xargs) python3'