This afternoon I happened to get tired of continually opening a terminal screen, firing up tmux, and proceeding to manually set up three separate panels of similar height arranged vertically on the screen. I figured that there had to be a way to do all that with a one-liner straight from the command line, and after some Googling and experimenting, I managed to get it figured out.
Typed that in and bam! I had three almost-equal panes in the terminal window, with the cursor sitting there in the first pane waiting for the next command.
tmux new-session \; split-window -p 66 \; split-window -p 50 \; select-pane -t 1