It might depend on which type of potatoes you are planting. I plant second earlies in two rows in a 1.2m bed and they do well. If I were planting first earlies I might try and fit in 3 rows, but perhaps such a bed might be too narrow for 2 rows of maincrops – but I’ve no experience other than second earlies so I’m only guessing…
I found earthing up 2 rows in a 1.2m bed in the traditional method quite tricky but adding compost to the bed around the plants as they grow as Charles seems to advocate makes the whole thing a lot easier.
I’m looking forward to my first new spuds of the year as I type!!!
Compostpope