De acordo com o link sugerido de @jasonwryan, criei o seguinte script para resolver o meu problema;
cd dir1
tmux new-window -a -n WinSplit
tmux new-session -d -s WinSplit
tmux selectp -t WinSplit
tmux split-window -v
cd dir2
tmux select-pane -U
tmux split-window -h
cd dir3
tmux split-window -v
#tmux select-layout even-vertical
tmux attach -t WinSplit
tmux bind -n M-Left select-pane -L
tmux bind -n M-Right select-pane -R
tmux bind -n M-Up select-pane -U
tmux bind -n M-Down select-pane -D