For empty plane, I'd rather be in economy than premium economy. Premium economy, only the middle two seats can combine, the rest of the arm-rests are not movable.
For econ 3 seat lie flat vs business class. Depends on how tall are you, and how much are you willing to spend for the extra room. I'm 6 feet tall (1.8m), 4 econ seats is just enough for me to lie flat, for 3 seats, I'll have to curl up my legs. I'm comfortable in the SQ A350 business seat.