Provide a way to switch between 2-way and 3-way diff
I often use Meld for ad-hoc visual comparisons between all sorts of items (e.g. logs, bits of code, text, ...). It is quite good and proficient at that, however it does not seem to provide an easy way to "convert" a 2-way comparison to a 3-way one. That would be very useful, as it's quite common for an initial 2-way comparison to lead to some modifications, and wanting to show a third option.