2+4+2 = 8, which is what the aircraft was designed for and is my favourite configuration when travelling with the wife as we get a couples chair to ourselves.
3+3+3 = 9, which is not what the aircraft was designed for and is a similar level of discomfort to a B777 in 10 across, and a B787 in 9 across.
Being an all economy configuration with the minimum legal number of cabin crew, the service won't be fantastic. In order to cram the seats in they need to be thin and expect them to be quite hard as there is little room for padding. Also expect long queues for the toilets.
It's like using a city bus designed for short rides around town on an 8 hour interstate run, yes it can be done but I wouldn't want to do it.